[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.

Before you request a review from someone, make sure to revise the following points:

• does the new code follow the style guide?
• do the test pipelines pass? (see guide on how to run pipelines for a merge request)
• 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)
• does your change affect public interfaces or behavior, or, does it introduce a new feature? If so, document the change in CHANGELOG.md.
• is the list of the header includes complete? ("include what you use")
• all files have to end with a \n character. Make sure there is no \ No newline at end of file comment in "Changes" of this MR.