TH2M changes necessary to use MFront material models

Main change is addition of missing interfaces for MFront internal variables extrapolation. Tested on a two-material model with internal states (same as for SmallDeformation).

First two commits handle two additional issues:

• Fix possible bug in state s0 initialization of material properties for MFront
• Exclude some computations in constitutive relations update and in Ehlers solid material model for initial (dt=0) case. This is temporary measure and requires splitting of the residuum and jacobian contributions.

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