for atmospheric plots.

More details:

*) Prep tools of reference data are updated so that they enable averaging
   for DJF/JJA. A switch do_djf/do_jja controles it in all 3 tools:
   tools/prep_era5_pyicon.py
   tools/prep_ceres_pyicon.py
   tools/prep_gpm_pyicon.py

*) drivers (atmospheric plots) are updated with the options do_djf/do_jja
   pyicon/quickplots/qp_driver.py
   pyicon/quickplots/qp_compare_atm_dwd.py

*) qp_timeseries and qp_timeseries_comp are updated with the options
   do_djf/do_jja in order to restrict the time averaging to DJF/JJA
   in yearly averages if requested.

*) additionally an option added to the drivers
   pyicon/quickplots/qp_driver.py
   pyicon/quickplots/qp_compare_atm_dwd.py
   to plot figures even if no reference data is defined/found. In
   this case bias & rmse plots are ignored.

*) For atmospheric plots we are using now time_at_end_of_interval=False
   as the ICON-NWP + mvstream outputs are so, that the actual monthly
   average is labeled for the first day of the month at 00 UTC.

[pyicon_dwd] Add some new plots: TOA net radiation flux, TOA outgoing short-wave flux. Change reference curves to black (from red).

    More details:

    *) CERES data are used as reference for
       -- TOA radiation fluxes
       -- total cloud cover

    *) GPM data are used as reference for
       -- total precipitation
       -- note, that for P-E plots, still ERA5 is used as reference,
          as evaporation is available only from ERA5

    *) If any of the 2 datasets (CERES, GPM) does not exist or not
       defined, the fall-back option is to use ERA5 for the above
       variables.

    *) Tools are added to prepare necessary CERES/GPM reference
       data sets:
       tools/prep_ceres_pyicon.py
       tools/prep_gpm_pyicon.py
       Thay are written in xarray and does the following:
       -- interpolates the data to arbitrary lat-lon
          resolution
       -- calculates time-mean maps for arbitrary time period
          for 2D fields
       -- calculates global-mean (cell-area weighted) times
          series

    *) pyicon/quickplots/qp_driver.py is updated so that CERES/GPM
       data are used as reference for bias & RMSE plots for the
       relevant variables (see above).

    *) pyicon/quickplots/qp_compare_atm_dwd.py is updated so that
       CERES/GPM data are used as reference for bias & RMSE plots
       for the relevant variables (see above).

    *) pyicon/quickplots/pyicon_quickplots.py is updated in order
       to show CERES/GPM data also in times series plots for the
       atmosphere. The option 'if use_tave_int_for_ts:' has been
       also rewritten wrt the slicing of the time series data
       given by --tave_int time bounds.

    *) tools/run_qp_driver_*_dwd.sh are updated by the path and
       file name of the CERES/GPM reference data.

    *) tools/run_qp_compare_*_dwd.sh are updated by the path and
       file name of the CERES/GPM reference data.

    ('part 2': all atmospheric plots created by qp_compare_atm_dwd.py)

    More details:

    *) pyicon/quickplots/qp_compare_atm_dwd.py is updated so that ERA5
       is used as reference for bias plots. Many new plots are added
       as compared to the previous case, where only a limited set
       of variables were available from ERA Interim the format that
       was eatable by pyicon.

    *) pyicon/quickplots/pyicon_quickplots.py is updated in order
       to show ERA5 data also in times series plots for the atmosphere.
       The changes are done under 'if use_tave_int_for_ts:' and
       'if fpath_ref_data_atm != ''' statements so that it does not
       change anything for ocean plots.

    *) tools/run_qp_compare_atm_dwd.sh is updated by the path and
       file name of the ERA5 reference data.

('part 1': all atmospheric plots created by qp_driver.py)

More details:

*) Tools are added to download and prepare necessary ERA5 data.
   tools/get_era5_pyicon.sh
   tools/prep_era5_pyicon.py
   The latter is written in xarray and does the following:
   -- interpolates the 0.25x0.25 data to arbitrary lat-lon
      resolution
   -- calculates time-mean maps for arbitrary time period
      for 2D fields and for 3D fields on selected pressure
      levels
   -- calculates zonal-mean time-mean lat-pressure
      cross-sections for arbitrary time period
   -- calculates global-mean (cell-area weighted) times
      series of selected variables

*) pyicon/quickplots/qp_driver.py is updated so that ERA5 is
   used as reference for bias plots. Many new plots are added
   as compared to the previous case, where only a limited set
   of variables were available from ERA Interim the format that
   was eatable by pyicon.

*) pyicon/quickplots/pyicon_quickplots.py is updated in order
   to show ERA5 data also in times series plots for the atmosphere.
   The changes are done under 'if use_tave_int_for_ts:' statements
   so that it does not change anything for ocean plots.

*) tools/run_qp_driver_*_dwd.sh are updated by the path and
   file name of the ERA5 reference data.

*) config_ckdtree/config_ckdtree_r2b4_atm_*_dwd.py are extended
   with a 1.5x1.5 grid. This is the resolution we use with R2B4.
   (also for a validation of R2B4 simulations, ERA5 data are
    interpolated to a 1.5x1.5 grid by tools/prep_era5_pyicon.py)

Additionally a bugfix is also included:

*) in qp_timeseries (pyicon/quickplots/pyicon_quickplots.py)
   the case have been fixed when use_tave_int_for_ts=True and
   vars_plot is a list of variables (and not just a single variable).
   times_plot had to be redefined within the loop over the vars_plot
   loop.

Updates to calculate_xr and addition of tests

See merge request !11

pyicon/pyicon_tb.py: Added function rename_dimension_names to allow for unique dimension nams in data and grid file.

pyicon/pyicon_tb.py: Moved rename_dimension_names to pyicon_tb.py. This however needs to be optimized.

pyicon/pyicon_calc_xr.py: Fixed typo and removed value error if dimensions of vector does not contain same dims as dz (e.g. to allow also time dim) in xr_edges2cell.

Fixed a typo in the pyicon_calculate_xr file

See merge request !9

pyicon/quickplots/qp_compare.py: Allow for optional reading of simulation namelist to avoid errors when namelist cannot be found.

pyicon/pyicon_tb.py: Added option for automatic fpath_ckdtree recognition. Added option in Triangulaiton to shift grid wrt dateline.

pyicon/pyicon_plotting.py: Added new plotting object to facilitate generation of axes and passing them to plotting routinse.