Fix/tpfa dispersion

What this MR does / why does DuMux need it:

There was a bug in the transmissibility calculation for tpfa disperion. This fixes this bug

Notes for the reviewer

TODO: insert text here

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• (if not applicable remove) are newly introduced or modified physical values/functions backed up with a scientific reference (including doi) in the docs?
• (if not applicable remove) if the examples are modified, is the documentation regenerated (using generate_example_docs.py)