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!

enabled multifield output for pn and mn solver. Unit tests not yet adapted!. PNSolver.solve still messy, needs cleanup!

added Option for Optimizer, added Optimizer, MNSolver compiles - not tested - base framework designed, added Entropy option and Entropy classes, added flux reconstruction step

harmonics unit test added; PN solver validation test added; deleted safety outputs for PN Solver; logger does not output stuff on screen (need to fix); Flux Jacobians for MN implemented