API change for the SyntheticControl experiment class

[drbenvincent]

• Towards Upgrade synthetic control to model multiple treated units #456
• This does not yet enable multiple treated units in synthetic control experiments. But it implements important prep work which will enable it. The changes focus on:
• API changes
• Bit of a spaghetti situation, but I had to switch from storing dataframes to xarray.DataArrays. This helps with the broadcasting. The model functions get varied input depending on the situation (e.g. experiment), so it was getting complicated. Xarray simplifies broadcasting in functions like PyMCModel.calculate_impact. (A bunch of time was used trying different solutions before it became clear that the xarray approach was the easiest).
• Because the API is changing, I elected to get rid of some legacy backward compatibility/ depreciation handling stuff. So when we do the next release, this will include breaking API changes and abandoning backward compatibility with an old API. I'm not fussed about this, we are currently at version 0.x, so people should expect the API to change until we reach 1.x.

Remaining taks:

• Check the multi cell geolift notebook is working as expected
• Fix failing doctest
• Resolve error below

Remaining bug, not captured by tests

In the scikit-learn synthetic control notebook, we are getting an error in the second part where we call

result = cp.SyntheticControl(
    df,
    treatment_time,
    control_units=["a", "b", "c", "d", "e", "f", "g"],
    treated_units=["actual"],
    model=LinearRegression(positive=True),
)

There is a broadcasting issue resulting in this:

So I need to think about if doing this (linear model with synthetic control experiment) even make sense. If it is, then I need to add a test to catch this error because it's not covered by tests currently, and fix it.

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📚 Documentation preview 📚: https://causalpy--460.org.readthedocs.build/en/460/

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Codecov Report

Attention: Patch coverage is 97.43590% with 2 lines in your changes missing coverage. Please review.

Project coverage is 94.40%. Comparing base (273daa2) to head (1f753e9).

Files with missing lines	Patch %	Lines
causalpy/experiments/synthetic_control.py	95.65%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #460      +/-   ##
==========================================
- Coverage   94.67%   94.40%   -0.27%     
==========================================
  Files          32       29       -3     
  Lines        2196     2075     -121     
==========================================
- Hits         2079     1959     -120     
+ Misses        117      116       -1     

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