Scaling for NUFFT operator

[chaithyagr]

I feel it will help a lot to have scaling coded up inside the nufft operator, similar to https://github.com/chaithyagr/tfkbnufft/blob/da8de17bc5cb738d11150662d0876bec9efb54d8/tfkbnufft/nufft/fft_functions.py#L159 and https://github.com/chaithyagr/tfkbnufft/blob/da8de17bc5cb738d11150662d0876bec9efb54d8/tfkbnufft/nufft/fft_functions.py#L197 as it will help a lot to make sure that Op and AdjOp can match the scales for ortho case.

[jmontalt]

Yes, I agree that a scaling option might be useful. This is easy to implement as a simple wrapper in Python, but in the long term we could incorporate it into the kernel and potentially get some extra performance.