[ff-pm-coupling] allow different discretization schemes

What this MR does / why does DuMux need it:

It allows to implement specializations for ff-pm coupling

Notes for the reviewer

I have made separate commits for the renaming such that the commit history is easier to read when looking at each of the commits. Before merging, we should squash these renaming commits in the corresponding subsequent commit.

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  • does the new code follow the style guide?
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  • is the code you changed and/or the new code you wrote covered in the test suite? (if not, extend the existing tests or write new ones)
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Edited by Martin Schneider

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