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.

doc/sphinx/lbmpy.bib

@@ -283,8 +283,9 @@ journal = {Communications in Computational Physics}
   publisher = {AIP Publishing},
   volume    = {27},
   number    = {8},
-  author    = {Wybe Rozema, Hyun J. Bae, Parviz Moin, Roel Verstappen},
+  author    = {Wybe Rozema and Hyun J. Bae and Parviz Moin and Roel Verstappen},
   title     = {Minimum-dissipation models for large-eddy simulation},
   journal   = {Physics of Fluids}
+}
 
 @Comment{jabref-meta: databaseType:bibtex;}