How to deal with multiple solid phases

The models 1pncmin and 2pncmin deal with possibly several solid phases. For example, there may be two solid phases, one referring to the original rock matrix and the other one to the precipitated salt. We should rethink how all of this is implemented and named.

For example, the number of solid phases currently is an enum in the fluid system. Why should a fluid system keep information about something solid? This is very counter-intuitive. Can we find a better place for this? Should we rename FluidSystem to ComponentSystem or PhaseSystem?

The current generic energy equations don't take into account the possibility of more than one solid phase. Should we add the capability there or should we treat this as corner cases and define specialized energy equations for 1pncmin and 2pncmin?