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/notebooks/06_tutorial_modifying_method_smagorinsky.ipynb

@@ -445,10 +445,10 @@
     }
    ],
    "source": [
-    "lbm_conifg = LBMConfig(stencil=Stencil.D2Q9, method=Method.MRT, force=(1e-6, 0),\n",
+    "lbm_config = LBMConfig(stencil=Stencil.D2Q9, method=Method.MRT, force=(1e-6, 0),\n",
     "                       force_model=ForceModel.LUO, relaxation_rates=[ω, 1.9, 1.9, 1.9])\n",
     "\n",
-    "method = create_lb_method(lbm_config=lbm_conifg)\n",
+    "method = create_lb_method(lbm_config=lbm_config)\n",
     "method"
    ]
   },