Fix fluctuating LB with zero centered pdf storage

Based on Frederik's work. I replaced an additional zeroth_moment_symbol by density_symbol and adapted the test such that the tolearnes on all fluctuating quantities scale with square root of system size.

tests/test_fluctuating_lb.py

@@ -14,7 +14,7 @@ from pystencils.enums import Target
 from pystencils.rng import PhiloxTwoDoubles
 
 from lbmpy.creationfunctions import *
-from lbmpy.forcemodels import Schiller 
+from lbmpy.forcemodels import Guo 
 from lbmpy.macroscopic_value_kernels import macroscopic_values_setter
 import numpy as np
 from lbmpy.enums import Stencil
@@ -137,7 +137,7 @@ def get_fluctuating_lb(
         weighted=True,
         zero_centered=zero_centered,
         relaxation_rates=rr_getter,
-        force_model=Schiller(force=force_field.center_vector),
+        force_model=Guo(force=force_field.center_vector),
         fluctuating={"temperature": kT},
         kernel_type="collide_only",
     )