Move files related to the PorePy bridge to a more suitable place

[keileg]

Currently, the files related to the PorePy bridge is available at tests/python/porepy. They should be moved, and possibly be made part of the python bindings of mpxa (this I have not thought carefully through).

Step 1: Figure out which files to move, and find a reasonable structure, likely in collaboration with copilot or similar.

[jwboth]

In #9: Moved to src and accessible as python modules via import mpxa next to the cpp methods.