Refactor codegen and jit modules: Initial file movements

src/pystencils/__init__.py

src/pystencils/backend/__init__.py

src/pystencils/backend/emission/base_printer.py

src/pystencils/backend/kernelcreation/context.py

src/pystencils/backend/kernelcreation/iteration_space.py

src/pystencils/backend/kernelfunction.py

src/pystencils/backend/memory.py

src/pystencils/backend/platforms/cuda.py

src/pystencils/backend/platforms/sycl.py

src/pystencils/backend/transformations/add_pragmas.py

src/pystencils/boundaries/boundaryhandling.py

src/pystencils/codegen/__init__.py

src/pystencils/codegen/config.py

src/pystencils/codegen/driver.py

src/pystencils/codegen/gpu.py

src/pystencils/codegen/kernel.py

src/pystencils/codegen/parameters.py

src/pystencils/codegen/properties.py

src/pystencils/codegen/target.py

src/pystencils/datahandling/__init__.py