Draft: [md][ffpnm] add diffusive energy flux

What this MR does / why does DuMux need it:

Fixes #1442 and is related to !3659 (merged) and #1435.

This introduces a the contribution of a diffusive flux to the total energy flux. Similar to what is done for Stokes-Darcy coupling.

TODO

• add this term in energyFlux_ for the composition case
• find and implement a test case, where this is relevant
• give option to disable or enable this term
• include in documentation (see also #1441)

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