introduce HipPlatform

src/pystencils/backend/platforms/__init__.py

src/pystencils/backend/platforms/cuda.py

src/pystencils/backend/platforms/generic_gpu.py

src/pystencils/backend/platforms/hip.py

src/pystencils/codegen/driver.py

src/pystencils/codegen/gpu_indexing.py