add dynamic array type

src/pystencils/__init__.py

src/pystencils/backend/kernelcreation/context.py

src/pystencils/backend/kernelcreation/freeze.py

src/pystencils/defaults.py

src/pystencils/field.py

src/pystencils/sympyextensions/typed_sympy.py

tests/frontend/test_field.py

tests/frontend/test_typed_sympy.py

tests/kernelcreation/test_iteration_slices.py

tests/nbackend/kernelcreation/test_context.py

tests/nbackend/kernelcreation/test_freeze.py