Implement pol_convention in pstokes module

[adeliegorce]

This PR introduces some changes to the pstokes module for two purposes

1. Take into account the new pol_convention attribute of UVData objects (see this PR) when constructing pseudo-Stokes polarizations whilst ensuring backwards compatibility (if there is no pol_convention, then there is a warning and avg is assumed). This is commit 6ffd0dc.
2. As per @jsdillon's request, create a new method called _combine_pol_arrays (final name still open to debate) which takes lists of data, flags, and nsamples arrays and combines them into pseudo-Stokes following the appropriate pol_convention (i.e. avg and sum). Although I see how useful this method can be, it does make the code slightly heavy so, reviewers, please let me know if you have any suggestions on how to make things cleaner. This is commit 2601670.
   I added new tests and modified existing ones accordingly.

[codecov]

Codecov Report

Attention: Patch coverage is 97.72727% with 1 line in your changes missing coverage. Please review.

Project coverage is 96.09%. Comparing base (8950367) to head (7a030d4).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
hera_pspec/pstokes.py	97.72%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #404   +/-   ##
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  Coverage   96.09%   96.09%           
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  Files          17       17           
  Lines        6121     6152   +31     
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+ Hits         5882     5912   +30     
- Misses        239      240    +1     

Flag	Coverage Δ
unittests	96.09% <97.72%> (+<0.01%)	⬆️

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[jsdillon]

Thanks @adeliegorce! A handful of comments, mostly related to documentation, as well as one substantial one about nsamples.

[adeliegorce]

Hi @jsdillon, I made the requested changes. Let me know if you're happy with the new version.

[steven-murray]

Thanks @adeliegorce -- I htink is almost ready to go, but does need a simple fix

[adeliegorce]

Done!

[jsdillon]

Just one suggestion that will hopefully avoid confusion in some weird edge cases

[jsdillon]

Looks good, thanks @adeliegorce!