introduce type atom wrapper. fix BC index dtype. Add CastFunc to freeze/typify.

src/pystencils/backend/kernelcreation/freeze.py

src/pystencils/backend/kernelcreation/typification.py

src/pystencils/boundaries/boundaryhandling.py

src/pystencils/boundaries/createindexlist.py

src/pystencils/sympyextensions/typed_sympy.py

tests/test_boundary.py