Merge branch 'threadSafety' into 'master'

doc/notebooks/02_tutorial_basic_kernels.ipynb

doc/notebooks/06_tutorial_phasefield_dentritic_growth.ipynb

doc/notebooks/demo_assignment_collection.ipynb

src/pystencils/boundaries/boundaryhandling.py

src/pystencils/config.py

src/pystencils/kernel_contrains_check.py

src/pystencils/kernelcreation.py

tests/test_jupyter_extensions.ipynb

tests/test_phasefield_dentritic_3D.ipynb

tests/test_vectorization_specific.py