Use closest normal for boundary index list with single_link

For creating the index list just the first stencil entry was taken which is a neighbour of the investigated cell if single_link=True. With this MR the discrete normal is calculated and the neighbouring cell in the normal direction is taken to build up the index array.

Furthermore, the computational cost of the python versions for create_boundary_index_list is reduced drastically because the iteration space is now restricted to the boundary cells and not the entire domain anymore.