Incremental nufft

[amaurea]

Mostly adds incremental_nufft, which lets nufft be used effectively in interpolator objects, where you use some resources to build the interpolator, but can then query it efficiently afterwards. Useful when you don't know how many points you will apply it to ahead of time, or if you need low-latency interpolation.

This also adds fourier interpolation as an option to some functions that use interpolation. This involved a breaking change in the interface. The argument mode → border, and mode now controls whether spline or fourier interpolation is done. I don't think many people have used this argument yet, but I'm still worried that it could break things, so please test your codes!

[codecov]

Codecov Report

Attention: Patch coverage is 31.32530% with 114 lines in your changes missing coverage. Please review.

Project coverage is 39.29%. Comparing base (b5c4c1d) to head (d64e1b7).

Files with missing lines	Patch %	Lines
pixell/fft.py	7.14%	39 Missing ⚠️
pixell/bench.py	0.00%	33 Missing ⚠️
pixell/utils.py	12.12%	29 Missing ⚠️
pixell/enmap.py	76.92%	12 Missing ⚠️
pixell/reproject.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #278      +/-   ##
==========================================
- Coverage   39.52%   39.29%   -0.24%     
==========================================
  Files          28       29       +1     
  Lines       10294    10401     +107     
==========================================
+ Hits         4069     4087      +18     
- Misses       6225     6314      +89     

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