Iteration Slices: Extended GPU support + bugfixes

.flake8

conftest.py

src/pystencils/backend/jit/gpu_cupy.py

src/pystencils/backend/kernelcreation/ast_factory.py

src/pystencils/backend/kernelcreation/iteration_space.py

src/pystencils/backend/kernelfunction.py

src/pystencils/backend/platforms/cuda.py

src/pystencils/backend/platforms/generic_gpu.py

src/pystencils/backend/platforms/platform.py

src/pystencils/backend/transformations/loop_vectorizer.py

src/pystencils/boundaries/boundaryhandling.py

src/pystencils/config.py

src/pystencils/datahandling/serial_datahandling.py

src/pystencils/kernelcreation.py

src/pystencils/simp/simplificationstrategy.py

src/pystencils/types/types.py

tests/fixtures.py

tests/kernelcreation/test_domain_kernels.py

tests/kernelcreation/test_functions.py

tests/kernelcreation/test_iteration_slices.py