Create regularized methods by default

This MR changes how create_lb_method handles relaxation rates when none are specified by the user. Also, the documentation of the handling of relaxation rates is clarified.

Previous Behaviour: When the user specifies no relaxation rates, all relaxation rates of the respective method are set to sp.Symbol('omega').

New Behaviour: When the user specifies no relaxation rates, the following default configurations are set up:

• SRT: A single relaxation rate omega is used
• TRT: A free rate omega and a bound rate computed from the magic number are used
• MRT/Central Moment/Cumulant: A regularized method is created; only the shear-viscosity governing relaxation rate is set to omega, all others are set to unity.

Rationale: Creating a method with just one relaxation rate as default behaviour is sensible. This relaxation rate is expected to govern the most important physics, i.e. the shear viscosity. This is achieved by regularized methods, while they also provide the highest level of stability. On the other hand, I observed that setting all relaxation rates to the same value omega will cause rapid instabilities in high Reynolds-number flows, whose cause is hard to track down if you are not aware how lbmpy determines relaxation rates.

Progress:

• Changed behaviour in creationfunctions.py
• Add test cases for desired behaviour
• Check and update documentation

lbmpy/creationfunctions.py

@@ -112,13 +112,19 @@ class LBMConfig:
     """
     Sequence of relaxation rates, number depends on selected method. If you specify more rates than
     method needs, the additional rates are ignored.
+
+    If no relaxation rates are specified, the parameter `relaxation_rate` will be consulted.
     """
     relaxation_rate: Union[int, float, Type[sp.Symbol]] = None
     """
-    For SRT, TRT and polynomial cumulant models it is possible to define
-    a single ``relaxation_rate`` instead of a list (Internally this is converted to a list with a single entry).
-    The second rate for TRT is then determined via magic number. For the moment, central moment based and the
-    cumulant model, it sets only the relaxation rate corresponding to shear viscosity, setting all others to unity.
+    The method's primary relaxation rate. In most cases, this is the relaxation rate governing shear viscosity.
+    For SRT, this is the only relaxation rate.
+    For TRT, the second relaxation rate is then determined via magic number.
+    In the case of raw moment, central moment, and cumulant-based MRT methods, all other relaxation rates will be
+    set to unity.
+
+    If neither `relaxation_rate` nor `relaxation_rates` is specified, the behaviour is as if 
+    `relaxation_rate=sp.Symbol('omega')` was set.
     """
     compressible: bool = False
     """
@@ -332,9 +338,11 @@ class LBMConfig:
             self.stencil = LBStencil(self.stencil)
 
         if self.relaxation_rates is None:
-            self.relaxation_rates = [sp.Symbol("omega")] * self.stencil.Q
+            #   Fall back to regularized method
+            if self.relaxation_rate is None:
+                self.relaxation_rate = sp.Symbol("omega")
 
-        # if only a single relaxation rate is defined (which makes sense for SRT or TRT methods)
+        # if only a single relaxation rate is defined,
         # it is internally treated as a list with one element and just sets the relaxation_rates parameter
         if self.relaxation_rate is not None:
             if self.method in [Method.TRT, Method.TRT_KBC_N1, Method.TRT_KBC_N2, Method.TRT_KBC_N3, Method.TRT_KBC_N4]: