Adds three classes for P2 Epsilon Operators
The new classes are children of the VectorToVectorOperator and called 1. P2ConstantEpsilonOperator 2. P2ElementwiseAffineEpsilonOperator 3. P2ElementwiseBlendingEpsilonOperator Operator 1 uses the FEniCS forms p2_(tet_)stokes_epsilon(_tet) while the other use corresponding HyTeG forms. These are the var variants and, thus, need a callback function for the variable vicosity. This is to be passed to the constructor of the operators. They will use it to generate objects for the sub-forms and pass them to the constructors of their Elementwise sub-operators. Additionally commit implements a P2ConstantFullViscousOperator. The latter is based on FEniCS forms. So it does support neither blending nor variable viscosity. Adds a ViscousOperatorsTest that checks generation of objects of the four new operator types for a 2D and a 3D setting.