Extend and fix documentation

.gitlab-ci.yml

doc/img/feature_optimization_overview.odp

doc/img/feature_optimization_overview.pdf

doc/img/feature_optimization_overview.png

doc/img/feature_optimization_overview.svg

doc/notebooks/00_tutorial_lbmpy_walberla_overview.ipynb

doc/sphinx/methods.rst

lbmpy/boundaries/boundaryconditions.py

lbmpy/methods/__init__.py

lbmpy/methods/centeredcumulant/__init__.py

lbmpy/methods/centeredcumulant/centered_cumulants.py

lbmpy/methods/centeredcumulant/centeredcumulantmethod.py

lbmpy/methods/centeredcumulant/force_model.py

lbmpy/methods/creationfunctions.py