Implemented the P1DGE constant vector Laplace operator.

Includes some fixes in the P0toP1 and P1toP0 mixed operators.
Also switched to P0Function for DGE.
31 jobs for kohl/stokes-eg in 30 minutes and 27 seconds (queued for 2 seconds)
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Name Stage Failure
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clang_13.0_mpionly_eigen_petsc_trilinos Pretest
[ 84%] Linking CXX executable ElementwiseOperatorPetscTest
[ 84%] Built target VertexDoFFunctionTest
[ 84%] Built target P2GMG3DConvergenceTest
[ 84%] Built target ElementwiseOperatorPetscTest
[ 84%] Linking CXX executable P2P1ElementwiseUzawaConvergenceTest
[ 84%] Built target P2P1ElementwiseUzawaConvergenceTest
make: *** [Makefile:141: all] Error 2
Cleaning up project directory and file based variables
ERROR: Job failed: exit code 1