Matrixfree and Vectorization of the Interior Penalty Method for Compressible Flow Solver

Building on the MatrixFree/vectorization work in collection this MR introduces an efficient matrix free type implementation of the IP diffusion operator in the compressible flow solver which can also be vectorised. On a TGV based on Hex elements this step was 5 times the cost of the advection step. After this merge it is reduced to 3 times and with vectorisation it can be reduced more. This makes a very notable speed up for this test