GMG-GC: implement free slip

[tjhei]

This might be working already, but it needs testing and removing this assert:

An error occurred in line <1577> of file </home/900358141/exp_asp/source/simulator/solver/stokes_matrix_free_global_coarsening.cc> in function
    void aspect::StokesMatrixFreeHandlerGlobalCoarseningImplementation<dimension, velocity_degree>::setup_dofs() [with int dim = 2; int velocity_degree = 2]
Additional information:
    This is not tested.