Fix RNG: use different key for each call

otherwise all random numbers drawn for one cell are equal

pystencils/rng.py

@@ -118,8 +118,12 @@ class PhiloxFourFloats(CustomCodeNode):
 
 
 def random_symbol(assignment_list, rng_node=PhiloxTwoDoubles, *args, **kwargs):
+    if 'keys' not in kwargs:
+        kwargs['keys'] = (0, 0)
+
     while True:
         node = rng_node(*args, **kwargs)
+        kwargs['keys'] = (kwargs['keys'][0] + 1, kwargs['keys'][1])
         inserted = False
         for symbol in node.result_symbols:
             if not inserted: