Add additional fast sky-based calibration to abscal module

[tyler-a-cox]

This PR adds a new sky-based calibration function to the abscal module. There are currently functions for performing traditional sky-based calibration (amp_logcal and phs_logcal), but both are quite slow as they involve matrix inversions. This new function, sky_calibration, implements the StEfCal algorithm (Salvini et al 2014) which scales as nants^2. Below is a time comparison between the existing abscal functions and the new method

And here is a comparison of the residuals of the two methods, showing nearly identical results for the two approaches

I also tested on H6C data using redundantly-calibrated, redundantly-averaged data as the sky model and found that the redundantly-calibrated data and sky-calibrated data agree quite well. Below are the delay-transformed redundantly calibration and sky-calibrated visibilities for a single baseline type.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.21%. Comparing base (3286222) to head (3447972).

Additional details and impacted files

@@            Coverage Diff             @@
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+ Coverage   97.19%   97.21%   +0.02%     
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  Files          31       31              
  Lines       11573    11657      +84     
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+ Hits        11248    11332      +84     
  Misses        325      325              

Flag	Coverage Δ
unittests	97.21% <100.00%> (+0.02%)	⬆️

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