refactor the uvbeam._convert_feeds_to_pols into a utility function

[bhazelton]

Description

Move an existing (private) function in uvbeam into a public utility function to make it more broadly useful (specifically in pyuvsim right now).

Motivation and Context

Types of changes

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

Checklist:

• I have read the contribution guide.
• 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 tutorial to highlight my new feature (if appropriate).
• I have added tests to cover my new feature.
• All new and existing tests pass.
• I have updated the CHANGELOG.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.93%. Comparing base (f1bb13c) to head (738c01c).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1529   +/-   ##
=======================================
  Coverage   99.93%   99.93%           
=======================================
  Files          63       63           
  Lines       21746    21752    +6     
=======================================
+ Hits        21731    21737    +6     
  Misses         15       15           

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