Use media in the two phase flow p-rho process

Replaces old fluid and porous media properties with their MPL counterparts.

I did not change the van Genuchten properties, since they are programmed in a slightly different way and would require re-verification of the tests; kept for another MR.

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2. Tests covering your feature were added? (Existing tests)
3. Any new feature or behavior change was documented? (Deleted)