Started refactor

src/pystencils/spatial_coordinates.py

src/pystencils/sympyextensions/__init__.py

src/pystencils/sympyextensions/assignment.py

src/pystencils/sympyextensions/assignment_collection.py

src/pystencils/sympyextensions/astnodes.py

src/pystencils/sympyextensions/bit_masks.py

src/pystencils/sympyextensions/math.py

src/pystencils/sympyextensions/simplifications.py

src/pystencils/sympyextensions/simplificationstrategy.py

src/pystencils/sympyextensions/subexpression_insertion.py

src/pystencils/sympyextensions/typed_sympy.py

src/pystencils/typing/cast_functions.py

src/pystencils/typing/typed_sympy.py

tests/nbackend/kernelcreation/test_domain_kernels.py

tests/nbackend/kernelcreation/test_index_kernels.py

tests/nbackend/test_basic_printing.py

tests/test_assignment_collection.py

tests/test_assignment_from_stencil.py

tests/test_astnodes.py

tests/test_bit_masks.py