Use units everywhere!

[jmbhughes]

PR summary

Uses units as extensively as possible

Todos

• clean up some of the transforms (delayed for another PR)
• improve test coverage (it's good enough)
• Remove CI for Python 3.10
• Update documentation and example notebook
• Check the polar keyword is set when starting resolve (delayed until another PR)
• Check the polar keyword is always as expected when starting a transform (delayed for another PR)
• Fix development instructions
• Update README to note new features
• Make a transform decorator to check NDCollection keys for proper inputs (and outputs?)

Test plan

Watch CI pass

Breaking changes

• Keys have all changed.
• Bumps min Python version.

Related Issues

Resolves #139

[codecov]

Codecov Report

Attention: Patch coverage is 91.75258% with 32 lines in your changes missing coverage. Please review.

Project coverage is 92.16%. Comparing base (d809388) to head (324f198).
Report is 4 commits behind head on main.

Files	Patch %	Lines
solpolpy/core.py	83.87%	10 Missing ⚠️
solpolpy/instruments.py	58.33%	10 Missing ⚠️
solpolpy/plotting.py	70.00%	6 Missing ⚠️
solpolpy/transforms.py	97.70%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #144      +/-   ##
==========================================
- Coverage   92.93%   92.16%   -0.77%     
==========================================
  Files           9        9              
  Lines         538      549      +11     
==========================================
+ Hits          500      506       +6     
- Misses         38       43       +5     

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