Fixed extraction of common srt eq term

Integration of zero-centered storage has broken a simplification step central to SRT/TRT methods. This MR fixes it.

lbmpy/methods/momentbased/momentbasedsimplifications.py

@@ -385,10 +385,15 @@ def __get_common_quadratic_and_constant_terms(cr: LbmCollisionRule):
         t = t.subs({ft: 0 for ft in sh['force_terms']})
 
     weight = t
+    weight = weight.subs(sh['density_deviation'], 1)
+    weight = weight.subs(sh['density'], 1)
 
     for u in sh['velocity']:
         weight = weight.subs(u, 0)
-    weight = weight / sh['density']
+    # weight = weight / sh['density']
     if weight == 0:
         return None
+
+    # t = t.subs(sh['density_deviation'], 0)
+
     return t / weight