Rework SGS models

doc/notebooks/06_tutorial_modifying_method_smagorinsky.ipynb

doc/sphinx/lbmpy.bib

src/lbmpy/__init__.py

src/lbmpy/creationfunctions.py

src/lbmpy/db.py

src/lbmpy/enums.py

src/lbmpy/turbulence_models.py

tests/test_json_serializer.py

tests/test_lbstep.py

tests/test_smagorinsky.py