Refactor reductionfunction mechanism

src/pystencils/backend/functions.py

src/pystencils/backend/kernelcreation/typification.py

src/pystencils/backend/platforms/cuda.py

src/pystencils/backend/platforms/generic_cpu.py

src/pystencils/backend/platforms/platform.py

src/pystencils/backend/platforms/sycl.py

src/pystencils/backend/transformations/select_functions.py

src/pystencils/codegen/driver.py