Merge branch 'extrapolation_outflow' into 'master'

.gitignore

.gitlab-ci.yml

doc/notebooks/02_tutorial_boundary_setup.ipynb

doc/sphinx/api.rst

doc/sphinx/boundary_conditions.rst

doc/sphinx/lbmpy.bib

lbmpy/advanced_streaming/utility.py

lbmpy/boundaries/__init__.py

lbmpy/boundaries/boundaryconditions.py

lbmpy/boundaries/boundaryhandling.py

...ests/advanced_streaming/test_advanced_streaming_noslip.py

lbmpy_tests/test_extrapolation_outflow.py