closed #22 by adding tests for mesh class; updated #24 by adding tests for academic lattice/checkboard testcases

Problemsetting

Closes #14 and #13

See merge request !8

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

Code refactoring

See merge request !7

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