WIP: [material][constant] Put volume work energy into storage term / internal energy

Harmonize enthalpy/internal energy of Constant component with the SimpleH2O implementation.

This is how it's done in SimpleH2O.

@holle Can you comment on this? Numerically it seems to make a difference if the consitutive law, i.e. dh = du + pdv is inserted for u or for h. It seems to me easier to account for the volume work in the volume integral.

As this has been only recently introduced in master !2421 (merged)/!2423 (merged) we can still change it easily.

Edit/Update: Constant component seems to be correct and SimpleH2O is the one that has a bug (wrong enthalpy and thus internal energy). So we shouldn't merge this but fix SimpleH2O instead.