Extended ExportVTK to multigroup writing. Restructured Solveroutput for PN and MN Solvers to Multigroup with cleaner code. Removed dFlux as Screenoutput. Adapted other solvers to maintain compatibility with the new ExportVTK format. _solverOutput is now legacy and will be removed in future commits!

updated extsource to be more flexible; replaced sparse matrix with dense matrix in the scattering kernels for now

added neumann type bc; added steady state convergence for test case validation(will be removed later on)

added problem class; crosssections are now cell specific; added checkerboard testcase; moved mpi sn to separate file; scattering implementation missing

fixed neighbor to normal sorting; fixed index error; all fluxes of cell a to b are equal to b to a everywhere

Code refactoring: Adaption to name conventions. Legacy Code Removed. Removed String array options in Config and replaced by safer vector<string> variants