Map which function are duplicated between PyPSA-Earth and PyPSA-Eur after 'Merge-March'

[pz-max]

In our developer meeting, we discussed that it would be quite nice to share functions between PyPSA-Eur and PyPSA-Earth. Currently, there is "Merge March" happening for which PyPSA-Eur will be merged with PyPSA-Eur-Sec. After this merge march event and after PyPSA-Earth is merged with PyPSA-Earth-Sec, we should figure out how to combine functions/ overlap them.