Add support for a single feed

[slosar]

Description

Relax the requirement that there are always 2 feeds leading to 4 polarizations. Now allow for a single feed. Still require that there are Nfeed^2 polarizations.

Needs changelog and docs updates...

Motivation and Context

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Reference simulation update or replacement
• [ ] Documentation change (documentation changes only)
• [ ] Version change
• [ ] Build or continuous integration change

Checklist:

For all pull requests:

• [x] I have read the contribution guide.
• [x] My code follows the code style of this project.

New feature checklist:

• [ ] I have added or updated the docstrings associated with my feature using the numpy docstring format.
• [ ] I have updated the documentation to highlight my new feature (if appropriate).
• [x] I have added tests to cover my new feature.
• [x] All new and existing tests pass.
• [ ] I have checked that I reproduce the reference simulations or if there are differences they are explained below (if appropriate). If there are changes that are correct, I will update the reference simulation files after this PR is merged.
• [ ] I have checked (e.g., using the benchmarking tools) that this change does not significantly increase typical runtimes. If it does, I have included a justification in the comments on this PR.
• [ ] I have updated the CHANGELOG.

[codecov[bot]]

Codecov Report

Merging #394 (c28cf25) into main (8092079) will decrease coverage by 0.04%. The diff coverage is 90.00%.

@@            Coverage Diff             @@
##             main     #394      +/-   ##
==========================================
- Coverage   99.34%   99.29%   -0.05%     
==========================================
  Files          13       13              
  Lines        2126     2130       +4     
==========================================
+ Hits         2112     2115       +3     
- Misses         14       15       +1     

Impacted Files	Coverage Δ
pyuvsim/uvsim.py	99.15% <83.33%> (-0.28%)	:arrow_down:
pyuvsim/antenna.py	96.07% <100.00%> (-0.08%)	:arrow_down:

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