Refactor Field Indexing Symbols

src/pystencils/__init__.py

src/pystencils/backend/arrays.py

src/pystencils/backend/kernelcreation/context.py

src/pystencils/backend/kernelcreation/freeze.py

src/pystencils/backend/kernelfunction.py

src/pystencils/datahandling/parallel_datahandling.py

src/pystencils/defaults.py

src/pystencils/field.py

src/pystencils/sympyextensions/math.py

src/pystencils/sympyextensions/typed_sympy.py

src/pystencils/typing.py

tests/nbackend/kernelcreation/test_context.py

tests/nbackend/kernelcreation/test_freeze.py