Optimization for GPU block size determination

docs/source/backend/gpu_codegen.md

docs/source/user_manual/gpu_kernels.md

src/pystencils/backend/platforms/cuda.py

src/pystencils/backend/platforms/sycl.py

src/pystencils/codegen/config.py

src/pystencils/codegen/driver.py

src/pystencils/codegen/gpu_indexing.py

src/pystencils/jit/gpu_cupy.py

src/pystencils/utils.py

tests/kernelcreation/test_gpu.py