4D image support checklist

[ptiede]

These are the things that need to be fixed to support multifrequency imaging.

• [ ] ContinuousImage really only works for spatial images. The summation for intensity_point is incorrect for Ti and Fr
• [x] FourierDualDomain DFT, FFT, and NFFT all have to be ported to loop over non-spatial dimensions
• [ ] uvgrid, padded_uvgrid and so forth only work for SpatialIntensityMaps
• [ ] InterpolatedModel only works with spatial intensitymaps
• [ ] Implement the clever caching for all the numerical FFT's (NFFT and DFT (done in #56) should be easiest)
• [ ] Maybe rework the type hierarchy to differentiate between purely spatial and more complicated models (traits?)
• [ ] PoincareSphere2Map needs to be updated to work with spatial and temporal models
• [ ] Make combinators (add and convolved model) work correctly with 4D images

This will be continually updated

[ptiede]

@rohandahale