Add GPU support

[Zethson]

Description of feature

• Skeleton & GPU CI
• fknni must support rapids & cupy arrays
• add a cupy option to blobs
• write a test which runs most of RSC directly
• Singledispatch scanpy functions to RSC
• Implement dynamic time warping kernel
• Rewrite existing numpy dense implementations with array API
• Add tests for GPU
• implement tutorial with GPU