Merged requested to merge mohr/ElementwiseOperators-mergeCandidate into master
this is a new attempt to have elementwise operators, i.e. operators that do not work with a stencil-based paradigm, but employ the local element matrices directly. Thus, the apply() will compute the effect of multiplying the element local DoFs with the local element matrix. Currently by setting the latter up, but in the future could also use the technique of Kronbichler et al.
This new attempt implements a P1ElementwiseOperator and a P2ElementwiseOperator for 2D and 3D. So far we can only apply the operator and perform Jacobi smoothing.