[ff] Use consistent averaged extrusion factor in momentum balance

What this MR does / why does DuMux need it:

The momentum balance of the freeflow model currently does not average extrusion factors across scvf for all flux contributions, violating flux conservation. This MR uses the same harmonic mean everywhere.

Notes for the reviewer

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• 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.