Disable cse_global on create_lb_collision_rule for Sympy < 1.2

I have this weird problem with cse_global and the fluctuating LB. It works fine with SymPy 1.2 and produces wrong temperatures with SymPy 1.1.1. I have bisected it down to this change: https://github.com/sympy/sympy/pull/13221. My patch disables cse_global if it's enabled and displays a warning on the incriminating SymPy version. I used Mathematica to plug the subexpressions back into the main expressions and get the same thing with both SymPy versions, so I have no idea what the actual problem is. Thus I can't rule out that it only occurs with the fluctuating LB. Until we know for sure, it seems safest to just disable cse_global on that old SymPy version entirely.

lbmpy/creationfunctions.py

@@ -349,7 +349,13 @@ def create_lb_collision_rule(lb_method=None, optimization={}, **kwargs):
         collision_rule = cse_in_opposing_directions(collision_rule)
     if cse_global:
         from pystencils.simp import sympy_cse
-        collision_rule = sympy_cse(collision_rule)
+        sp_ver = sp.__version__.split('.')
+        if int(sp_ver[0]) > 1 or int(sp_ver[1]) > 1:
+            collision_rule = sympy_cse(collision_rule)
+        else:
+            import warnings
+            warnings.warn('SymPy versions below 1.2 may perform common subexpression elimination incorrectly on LB'
+                          'collision rules. Ignoring cse_global.')
 
     if params['output'] and params['kernel_type'] == 'stream_pull_collide':
         cqc = lb_method.conserved_quantity_computation