Separate InterruptedTimeSeries and SyntheticControl classes, update uml

[drbenvincent]

Closes #457

As well as running the tests locally, I also ran all relevant notebooks to check for no errors. But I'm not updating the notebooks here because the API is unchanged and the results should be identical

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📚 Documentation preview 📚: https://causalpy--458.org.readthedocs.build/en/458/

[codecov]

Codecov Report

Attention: Patch coverage is 97.48428% with 4 lines in your changes missing coverage. Please review.

Project coverage is 94.66%. Comparing base (4227edf) to head (867d93b).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
causalpy/experiments/interrupted_time_series.py	97.01%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #458      +/-   ##
==========================================
+ Coverage   94.53%   94.66%   +0.13%     
==========================================
  Files          31       32       +1     
  Lines        2068     2195     +127     
==========================================
+ Hits         1955     2078     +123     
- Misses        113      117       +4     

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