[pyicon_dwd] Add options to plot DJF (NH Winter) and JJA (NH Summer) means

             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.