[FIX] PSM generation

Due to !176 (merged) an assertion was introduced in the creation of the symbolic relaxation matrix. The assertion tests that all entries of the symbolic relaxation matrix are sympy symbols. However, when a relaxation_rates_modifier this might not be the case.

This MR fixes the problem and introduces a test case with the psm method

src/lbmpy/methods/abstractlbmethod.py

@@ -124,7 +124,8 @@ class AbstractLbMethod(abc.ABC):
         substitutions = [Assignment(e[1], e[0]) for e in subexpressions.items()]
         if relaxation_rates_modifier is not None:
             symbolic_relaxation_rates = [r * relaxation_rates_modifier for r in symbolic_relaxation_rates]
-        for srr in symbolic_relaxation_rates:
-            assert isinstance(srr, sp.Symbol)
+        else:
+            for srr in symbolic_relaxation_rates:
+                assert isinstance(srr, sp.Symbol)
 
         return substitutions, sp.diag(*symbolic_relaxation_rates)