[pyicon_dwd] Difference plots for the atmosphere, area weighted horizontal mean of fields, new fields for atmosphere plots, etc.

The most important changes included:

-- diff plots (diff, RMSE, RMSE-diff) for the atmosphere are introduced in order to compare pairs of experiments. This is based on tools/run_qp_compare_atm_dwd.sh and pyicon/quickplots/qp_compare_atm_dwd.py. The latter uses the new function qp_timeseries_comp from pyicon/quickplots/pyicon_quickplots.py.

-- for the do_write_data_range option the simple horizontal mean/stdev calculation is replaced by an area-weighted horizontal mean/stdev calculation. See the new function calc_grid_area in pyicon/pyicon_plotting.py, which is called from hplot_base.

-- ts_gmean* plots limited to the time period defined by --tave_int (both in single exp and compare mode). This consists of a calculation of lend based on t2 (if not defined as argumrent) in qp_timeseries (and qp_timeseries_comp) in pyicon/quickplots/pyicon_quickplots. I did this because some of my colleagues needed a simple way to enable plotting time-series of sub-periods of the full experiment period.

-- new maps of fields added to the atmosphere (both single exp and comparison mode): TOA sw net flux map, TOA lw net flux map, Surface sensible heat flux map, Surface latent heat flux map, SST, seaice-fraction (the latter two might seem silly as we already have that for the ocean, but we wanted to have these also in AMIP type of runs to see how the prescribed ocean looks like beneath our atmo simulation).

-- I revitalized the link "pyicon documentation", which in our case was empty. Therefore, I created doc/doc_dwd/html/index.html, which is copied to the needed link when the QP runs. This is saved with a do_conf_dwd statement. I hope I did not overlook a better way to do this.

-- I created add_info_comp in pyicon/quickplots/pyicon_quickplots.py in order to give sense to he "additional info" link in case of diff plots (to have the namelists there from both experiments) I also added (both in add_info and add_info_comp a search for nml.atmo*, nml.ocean*, so that these are also clickable under the link "aditional info".