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.