Cleanup/pnm 2pnc

What this MR does / why does DuMux need it:

This Merge request is a followup of !3197 (merged). Here some changes has been done to the boundary conditions to make them more organized and clear and also the source term has been removed to make the interpretation of the results easier. Other than that, some cleanups has been done.

Fixes #1361 (closed)

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