Performs minor code cleanup
Commit removes:
- left over conflict markers in P1ConstantOperator.hpp
- unimplemented private methods assembleStencils() and assembleStencils3D() of P2ConstantOperator class
- P2Elements3D::calculateLocalP2StiffnessMatrix(); this was no longer called by anyone and is superseded by the integrate() methods of P2FenicsForm