Adds further P2 assembly code for general_forms integration.
- Constructor of EdgeDoFOperator now calls existing method assembleEdgeToEdgeStencils() again for the 3D case. - Implemented for P1ToP2FenicsForm and P2ToP1FenicsForm the following methods * computeLocalStiffnessMatrix() ... for the 3D case * integrate() ... version that gets called by P1ConstantOperator * integrate() ... version that gets called by P2Elements3D