Add ability to simulate visibilities used a user-supplied beam basis and basis coefficients

[tyler-a-cox]

This PR builds on the per-antenna beam simulation functionality introduced in #63.

Currently, the scaling of fftvis with the number of unique beams is computationally expensive, scaling between ~N_beams and N_beams^2. This creates a performance bottleneck when running simulations with large arrays of unique beams.

This PR introduces the option to simulate visibilities using a reduced basis formed from a stack of unique beams. Assuming the beams used for simulation are largely similar to one another, we can form a basis using the SVD/PCA of the beam stack to reduce the effective number of unique beams passed to fftvis. This reduces the number of required NUFFTs and beam interpolation from ~N_beams^2 down to N_eigenbeams^2.

Here, we introduce a new function, which accepts a list UVBeam objects and returns a list of "eigenbeams" (also stored as UVBeam objects) and beam coefficients (of size (nants, ncoefs, nfreqs)) that map the eigenbeams to the input UVBeam objects. These beams/coefficients can then be passed to fftvis.simulate_vis via the beam parameter and the new eigenbeam_coefs parameter to produce visibilities with per-antenna beams.

[codecov]

Codecov Report

❌ Patch coverage is 53.38346% with 62 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.40%. Comparing base (1a76fa7) to head (655f10e).

Files with missing lines	Patch %	Lines
src/fftvis/cpu/cpu_simulate.py	62.22%	28 Missing and 6 partials ⚠️
src/fftvis/core/beam_basis.py	17.24%	24 Missing ⚠️
src/fftvis/wrapper.py	66.66%	2 Missing and 2 partials ⚠️

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HEAD has 1 upload less than BASE

Flag	BASE (1a76fa7)	HEAD (655f10e)
unittests	6	5

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #66      +/-   ##
==========================================
- Coverage   95.22%   87.40%   -7.83%     
==========================================
  Files          18       19       +1     
  Lines         566      635      +69     
  Branches       86       97      +11     
==========================================
+ Hits          539      555      +16     
- Misses         17       66      +49     
- Partials       10       14       +4     

Flag	Coverage Δ
unittests	87.40% <53.38%> (-7.83%)	⬇️

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