Add support for upcoming memory efficient (cu)fiNUFFT

[chaithyagr]

In flatironinstitute/finufft#633 we see that flatiron institue folks have brought in a feature we had been requesting:

flatironinstitute/finufft#571

i.e. to have a single plan to do both forward and ajoint NUFFT.

This support can be updated into our codebase for future direct use by users.