Merged requested to merge he66coqe/walberla:codegen-complex-geometry-freeslip into master
Add FreeSlipAdditionalDataHandler. It determines the discrete normal direction from the flag field. This makes it possible to use the FreeSlip boundary condition from lbmpy with complex geometries. Implementation closely follows FreeSlip.h
Note that this MR only works together with pycodegen/lbmpy!115 (merged).