Modified cam clay version with const E: clean-up, bug-fix, renaming

Changes

• redundancy removed: only volume ratio evolution left, no porosity anymore
• minimum value for the pre-consolidation pressure introduced => yield surface cannot degenerate to point anymore
• elasticity tensor inversion simplified (required for consistent initial state with absolute elastic formulation)

Fixes

• workaround for state variable initialization in MFront removed
• bugfix algorithmic tangent, only relevant for theta<1

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