Add FRF noise covariance calculation library

[mwilensky768]

This branch adds some noise propagation functions for the fringe rate filter + coherent averaging steps.

I've added the basic functions that are demonstrated in the single baseline power spectrum notebook, though they've been modified here to be slightly more general since the notebook versions used notebook-specific info. This PR and the corresponding one on that repo should probably coordinate.

I still need to add unit tests and think I should add a wrapper that goes from ee + nn autos/crosses all the way to a pI time-time cov. per freq so that there is a clearly implemented "pipeline" somewhere, though maybe that should just live in another repo as part of actual pipeline code.

[codecov]

Codecov Report

Attention: Patch coverage is 12.67606% with 62 lines in your changes missing coverage. Please review.

Project coverage is 96.62%. Comparing base (0529798) to head (e6d8dca).

Files with missing lines	Patch %	Lines
hera_cal/frf.py	8.95%	61 Missing ⚠️
hera_cal/noise.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #977      +/-   ##
==========================================
- Coverage   97.16%   96.62%   -0.54%     
==========================================
  Files          31       31              
  Lines       11064    11133      +69     
==========================================
+ Hits        10750    10757       +7     
- Misses        314      376      +62     

Flag	Coverage Δ
unittests	96.62% <12.67%> (-0.54%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.