pyicon plots now return axes and other plot data when accessed using the pyicon xarray accessor. This behaviour is more consistent with xarray's plotting functions and the pyicon plotting functions

A fix for the default naming of dimensions such as "nX"

See merge request m300602/pyicon!13

# Conflicts:
#   pyicon/pyicon_calc_xr.py

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

See merge request m300602/pyicon!12

fix import errors

See merge request m300602/pyicon!15