Added default implementations for planestrain, planestress and 1d for PK-stress computations. PlaneStress and 1D is solved iteratively.
Removed unnecessary MatResponseForm from all functions related to 1PK. Changed default implementation for MisesMat to overload _3d version so that PlaneStrain, PlaneStress and 1D behavior is automatically supported.
Commented out 1PK-code that doesn't make much sense (yet?)