Add support for arbitrary polarization angles specification and npol system

[jmbhughes]

PR summary

allows arbitrary angles to be passed closing #137

Todos

• add an arbitrary path test
• fix representation of npol to be an arbitrary number of angles
• remove hardcoded stereo offset angles for npol
• improve docs
• represents transform systems as lower cased strings internally

Test plan

Make sure CI passes

Breaking changes

None

Related Issues

Closes #137

[codecov]

Codecov Report

Attention: Patch coverage is 94.77612% with 7 lines in your changes missing coverage. Please review.

Project coverage is 92.93%. Comparing base (cbf4291) to head (9a37cbe).

Files	Patch %	Lines
solpolpy/instruments.py	69.23%	4 Missing ⚠️
solpolpy/core.py	93.75%	2 Missing ⚠️
solpolpy/polarizers.py	98.55%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #138      +/-   ##
==========================================
- Coverage   93.13%   92.93%   -0.21%     
==========================================
  Files           9        9              
  Lines         481      538      +57     
==========================================
+ Hits          448      500      +52     
- Misses         33       38       +5     

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