Rework SGS models

This merge request generalises the definition of subgrid-scale (SGS) models.
Until now, only the Smagorinsky model could be defined in the LBMConfig (by setting True or the model constant).
Now, a SGS model can be provided by an enumeration or as a tuple of the SGS enum and its model constant.

In the same time, this MR adds an implementation for the QR model, an isotropic minimum-dissipation model.

Furthermore, the possibility to output the eddy-viscosity directly from the SGS model calculation is added. This facilitates the post-processing and validation of turbulent flows.

src/lbmpy/creationfunctions.py

@@ -241,6 +241,10 @@ class LBMConfig:
     A pystencils Field can be passed here, where the calculated free relaxation rate of
     an entropic or subgrid-scale method is written to
     """
+    eddy_viscosity_field: Field = None
+    """
+    A pystencils Field can be passed here, where the eddy-viscosity of a subgrid-scale model is written.
+    """
     subgrid_scale_model: Union[SubgridScaleModel, tuple[SubgridScaleModel, float], tuple[SubgridScaleModel, int]] = None
     """
     Choose a subgrid-scale model (SGS) for large-eddy simulations. ``omega_output_field`` can be set to
@@ -727,7 +731,8 @@ def create_lb_collision_rule(lb_method=None, lbm_config=None, lbm_optimisation=N
             model_constant = lbm_config.subgrid_scale_model[1]
 
         collision_rule = add_sgs_model(collision_rule=collision_rule, subgrid_scale_model=sgs_model,
-                                       model_constant=model_constant, omega_output_field=lbm_config.omega_output_field)
+                                       model_constant=model_constant, omega_output_field=lbm_config.omega_output_field,
+                                       eddy_viscosity_field=lbm_config.eddy_viscosity_field)
 
     if 'split_groups' in collision_rule.simplification_hints:
             collision_rule.simplification_hints['split_groups'][0].append(sp.Symbol("sgs_omega"))