Fix fftvis beamidx

[steven-murray]

Pass the beam_idx parameter through to FFTVis.

Also adds a test that compares against pyuvsim using multiple beams. Note that for some reason if polarized=False this test fails. I made #414 to track this bug.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.65%. Comparing base (b36e109) to head (b614ace).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #415      +/-   ##
==========================================
+ Coverage   93.63%   93.65%   +0.02%     
==========================================
  Files          25       25              
  Lines        3345     3358      +13     
  Branches      555      555              
==========================================
+ Hits         3132     3145      +13     
  Misses        112      112              
  Partials      101      101              

Flag	Coverage Δ
unittests	93.65% <100.00%> (+0.02%)	⬆️

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[tyler-a-cox]

This is looking good to me! We should probably make a note to fix the polarized=False case at some point though