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.

requested review from @holzer

assigned to @schottenhamml

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• b4a1f61e - Fix flake8.

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• 78a669f4 - Fix bibtex entry.

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• 907954e0 - Fix bibtex entry. Try 2.

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• a814efb1 - Fix bibtex entry. Try 3.

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approved this merge request

merged

mentioned in commit e57075a0