Add unit conversion API

[znicholls]

Description

Checklist

Please confirm that this pull request has done the following:

• Tests added
• Documentation added (where applicable)
• Changelog item added to changelog/

[codecov]

Codecov Report

❌ Patch coverage is 98.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.04%. Comparing base (ae6748c) to head (07b7b73).
⚠️ Report is 115 commits behind head on main.

Files with missing lines	Patch %	Lines
src/pandas_openscm/accessors.py	83.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #23      +/-   ##
==========================================
+ Coverage   96.98%   97.04%   +0.06%     
==========================================
  Files          27       28       +1     
  Lines        1358     1455      +97     
  Branches      153      168      +15     
==========================================
+ Hits         1317     1412      +95     
- Misses         23       25       +2     
  Partials       18       18              

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