Fix uninserted aliases in cumulant collision rules + new test cases

Too few insertion passes where run on the cumulant kernel. Fixed that, and introduced a test case to check for desired simplification behaviour in MRT methods.

lbmpy/simplificationfactory.py

@@ -92,6 +92,7 @@ def _cumulant_space_simplification(split_inner_loop):
     s.add(expand_post_collision_central_moments)
     s.add(insert_aliases)
     s.add(insert_constants)
+    s.add(insert_aliases)
     s.add(add_subexpressions_for_divisions)
     s.add(add_subexpressions_for_constants)
     if split_inner_loop: