MPNC "MaterialLaw API" creates unnecessary dependencies

The concept used by the "TwoPAdapter" seems to be something that belongs in the volume variables only. Currently the spatial params need to know about the fluid system. However that knowledge is available in the volume variables and computing pressure as the MPNC API seems to do is also the task of the volume variables.

Can we move the code somehow to the volume variables and design the interface in a way that knowledge of the fluid system and fluid state is not needed in the spatial params?