Restructure Vcycle and Diffusion Solver

[LinhWuerzburger]

In VCycleMG.cpp is duplicate code, which is from the respective diffusion solver. To prevent duplicate code, investigate whether it is possible to restructure the Diffusion Solver Jacobi as well ColoredGaussSeidel as such that the error/residuum calculation doesn't need to be done in the VCycleMG class.

[LinhWuerzburger]

regarding the diffusion solver Jacobi: as you can see in the picture, the function for multigrid and non multigrid are almost the same. needs only a bit a restructuring.