Various GPU-related and some general fixes.

conftest.py

src/pystencils/backend/jit/__init__.py

src/pystencils/backend/jit/gpu_cupy.py

src/pystencils/backend/jit/jit.py

src/pystencils/backend/jit/legacy_cpu.py

src/pystencils/backend/kernelcreation/analysis.py

src/pystencils/backend/kernelcreation/ast_factory.py

src/pystencils/backend/kernelcreation/context.py

src/pystencils/backend/kernelcreation/freeze.py

src/pystencils/backend/kernelcreation/iteration_space.py

src/pystencils/backend/platforms/cuda.py

src/pystencils/backend/platforms/sycl.py

src/pystencils/config.py

src/pystencils/gpu/periodicity.py

src/pystencils/kernel_wrapper.py

src/pystencils/kernelcreation.py

src/pystencils/types/parsing.py

tests/nbackend/kernelcreation/test_gpu.py

tests/nbackend/kernelcreation/test_index_kernels.py

tests/nbackend/kernelcreation/test_iteration_space.py