Add a bunch of utils functions for reconstruction

[chaithyagr]

This includes (with GPU support):

• Compress Coils
• Add Phase to k-space data

[chaithyagr]

@paquiteau , your call, what are the stuff you think should be in mri-nufft here?
To me, both belong to pysap-mri as it is specific to k-space data and not NUFFT operator. However, just keeping in you in loop

[paquiteau]

I think it's fine if its stays in pysap-mri. If you consider torch support or something alike it might be a better fit in mri-nufft.