Ins compatibility

[mwilensky768]

This PR fixes a critical issue, which is that the UVFlag class from pyuvdata has shifted enough that the child class, INS is no longer compatible. In particular, the UVFlag constructor call signature has 0 positional arguments, which is a major change compared to when INS was originally defined. This results in a complete inability to read objects, since UVFlag.read calls UVFlag.init() with an empty signature before reading. This basic functionality is restored, along with a significant but minor refactor.

Description

The INS constructor had to be completely refactored. In particular, I have allowed for an empty call signature. What this does is preserves some INS-specific attributes and then calls the UVFlag constructor using the arguments that are passed. If the call signature is empty, this will empty the object except for the INS-specific attributes that are required to properly calculate e.g. the metric_ms attribute. This happens whenever UVFlag.read is called, in which case there is no way to preserve these attributes if read is called from a previously instantiated object. For this reason, SSINS raise a NotImplementedError if the user tries to call INS.read from an object that is already full constructed.

To handle a non-empty call signature, we overrode read and from_uvdata. These basically start with some checks to establish whether these are safe calls by the user, call the UVFlag method, and then do the INS extras, preserving INS-specific attributes where necessary. We made a few ("private") helper methods to make this type of thing a bit more modular.

We also refactored the SS class a bit, mostly to keep up with some new required attributes in UVData objects. Since it was related to I/O, I decided to include it in this PR even though it's a little out of scope technically.

This all is API-preserving, although a particular version of pyuvdata will be required if reading files that were written without telescope names (mostly very old files).

Checklist:

Main Code Body Checklist:

• Add or update docstring related to code change
• Add a unit test that verifies the efficacy of the code
• Write a tutorial if the feature would be useful to others to use
• Update CHANGELOG.md

Scripts Checklist:

• Add useful help strings for any arguments that are parsed
• Manually check that the script runs and gives proper results
• Update CHANGELOG.md

[codecov-commenter]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (e95b824) 100.00% compared to head (209fce0) 99.66%.

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Additional details and impacted files

@@             Coverage Diff             @@
##            master     #121      +/-   ##
===========================================
- Coverage   100.00%   99.66%   -0.34%     
===========================================
  Files            8        8              
  Lines          881      908      +27     
===========================================
+ Hits           881      905      +24     
- Misses           0        3       +3     

Files	Coverage Δ
SSINS/sky_subtract.py	99.41% <91.66%> (-0.59%)	⬇️
SSINS/incoherent_noise_spectrum.py	99.31% <97.70%> (-0.69%)	⬇️

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