The propertie belongs into the material models.
Computations in near future will require only constant
solid phase volume fraction, so the switch is added
to separate constant and non-constant parts.

Needed for correct computation of thermal strains
and solid material models' updates.

Stresses and strains are now updated together with all the other
constitutive variables.

Function is not needed.

First step to separte the behaviour from state.

Saves a little bit of typing.

and replaces it with interpolated version

Same values are stored in the integration point data.