Started refactor

src/pystencils/fd/derivative.py

src/pystencils/fd/spatial.py

src/pystencils/field.py

src/pystencils/functions.py

src/pystencils/integer_set_analysis.py

src/pystencils/kernel_contrains_check.py

src/pystencils/kernel_decorator.py

src/pystencils/kernelcreation.py

src/pystencils/nbackend/kernelcreation/kernelcreation.py

src/pystencils/node_collection.py

src/pystencils/old/backends/__init__.py

src/pystencils/old/backends/arm_instruction_sets.py

src/pystencils/old/backends/cbackend.py

src/pystencils/old/backends/cuda_backend.py

src/pystencils/old/backends/dot.py

src/pystencils/old/backends/json.py

src/pystencils/old/backends/ppc_instruction_sets.py

src/pystencils/old/backends/riscv_instruction_sets.py

src/pystencils/old/backends/simd_instruction_sets.py

src/pystencils/old/backends/x86_instruction_sets.py