expose architecture of Materials

[WilkAndy]

This is a hard one... Just looking at an input file, you really have no idea of how the Material+Kernel stuff works. You don't know that PorousFlowMaterial1PhaseP_VG provides a pore pressure and saturation, and that feeds into XXX which feeds into a Joiner, which feeds into a Kernel.

Somehow it'd be good to make the inputs and outputs of each class more obvious so people can see the flow of information.

Perhaps with an Action it will be less important?