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
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