Added central moments functionality to lbmpy

This merge request will add central moment functionality to lbmpy (shift_matrix, get_central_moments). Remaining TODOs:

• The Demo notebook can not be executed at the moment since the includes are not correct.
• Integrate central moments fully into lbmpy. This means something like this should be possible create_lb_method(..., central_moments=True).
• Unit tests for the get_shift_matrix function.
• Physical test case the central moments.