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/enums.py

@@ -230,9 +230,9 @@ class SubgridScaleModel(Enum):
     """
     SMAGORINSKY = auto()
     """
-    See :function:`lbmpy.turbulence_models.add_smagorinsky_model`
+    See :func:`lbmpy.turbulence_models.add_smagorinsky_model`
     """
     QR = auto()
     """
-    See :function:`lbmpy.turbulence_models.add_qr_model`
+    See :func:`lbmpy.turbulence_models.add_qr_model`
     """