make installable via `pip`

I wanted to use pyicon in an existing conda environment and could install pyicon via pip.

Goal: install with pip install . and therefore pip install https://gitlab.dkrz.de/m300602/pyicon

Consequence: Don't require import sys; sys.path.insert(0,'/path/to/pyicon')

conda install -c conda-forge xarray cartopy dask matplotlib
pip install .

• add setup.py
• cartopy is a messy dependency, much easier installing beforehand with conda, see https://stackoverflow.com/questions/53697814/using-pip-install-to-install-cartopy-but-missing-proj-version-at-least-4-9-0