[pm][nc] Account for useMoles for compositional dispersion

What this MR does / why does DuMux need it:

Compositional dispersion didn't account for useMoles by scaling the mass/mole (based on reference system) fluxes with molar masses to obtain the required mass/mole flux (based on useMoles). The flux was only correct if the two were the same.

Notes for the reviewer

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