Interpolation

[herjy]

This Pr includes the following changes :

• Fourier objects now have shape and axes in the key of the transforms instead of having axes as an attribute.
• New mk_shifter function creates vectors for shifts in Fourier space and caches them.
• sinc_interp now works for rotated frames in its "fast" version
• prox_kspace_symmetry now uses Fourier instances with cached shifting vectors
• a single sinc_shift method now allows to compute fast resampling in aligned AND rotated frames
• 'LowResObservation' now uses Fourier objects.