Add functions for flagging auto-correlations

[tyler-a-cox]

This PR implements new RFI flagging functions for auto-correlation using channel differencing and DPSS filtering from @AaronParsons fastcal notebook.

[codecov[bot]]

Codecov Report

Merging #423 (5b1ecba) into main (4f82bc5) will increase coverage by 0.00%. The diff coverage is 97.01%.

:exclamation: Current head 5b1ecba differs from pull request most recent head b6169ff. Consider uploading reports for the commit b6169ff to get more accurate results

@@           Coverage Diff           @@
##             main     #423   +/-   ##
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  Coverage   96.99%   96.99%           
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  Files          10       10           
  Lines        3465     3532   +67     
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+ Hits         3361     3426   +65     
- Misses        104      106    +2     

Impacted Files	Coverage Δ
hera_qm/xrfi.py	99.59% <96.61%> (-0.19%)	:arrow_down:
hera_qm/ant_class.py	97.35% <100.00%> (+0.11%)	:arrow_up:

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[tyler-a-cox]

Thanks for all of your comments @jsdillon! This should be ready for another look. I also added a function which classifies antennas by their flag fraction relative to the array-wide flag fraction. I added it to xrfi, but a better place for it may be ant_class. Let me know what you think!

[tyler-a-cox]

Thanks for the notes @jsdillon. I think this should be the last time you need to look at this. If I missed anything, let me know!