[pyicon_dwd] Use ERA5/CERES/GPM data as reference for atmospheric plots

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

*) pyicon/quickplots/qp_driver.py is updated so that ERA5/CERES/GPM are 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/CERES/GPM data also in times series plots for the atmosphere. The changes are done under "if fpath_ref_data_atm != '':" 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/CERES/GPM 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)