Merge branch 'bc-diffusion-dirichlet' into 'master'

lbmpy/boundaries/__init__.py

lbmpy/boundaries/boundaryconditions.py

lbmpy/maxwellian_equilibrium.py

lbmpy_tests/test_boundary_handling.py

lbmpy_tests/test_diffusion.py

lbmpy_tests/test_vectorization.py