various refactorings and quick-fixes

src/pystencils/backend/jit/legacy_cpu.py

src/pystencils/backend/kernelcreation/context.py

src/pystencils/backend/kernelcreation/freeze.py

src/pystencils/boundaries/boundaryconditions.py

src/pystencils/boundaries/boundaryhandling.py

src/pystencils/boundaries/inkernel.py

src/pystencils/field.py

src/pystencils/gpu/__init__.py

src/pystencils/gpu/gpu_array_handler.py

src/pystencils/integer_functions.py

src/pystencils/kernelcreation.py

src/pystencils/old/gpu/__init__.py

src/pystencils/rng.py

src/pystencils/sympyextensions/__init__.py