Expand data validation to also cover the scikit-learn experiment classes

[drbenvincent]

• Closes Add input validation to experiment objects #78
• We currently have _input_validation methods in just the pymc experiment classes
• This PR extracts these out into new Mixin classes in data_validation.py
• Now we can use the same data validation code for both the pymc and the scikit-learn versions of the experiment classes.

[codecov]

Codecov Report

Attention: Patch coverage is 97.64706% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 77.06%. Comparing base (67d9e0e) to head (6574481).

Files	Patch %	Lines
causalpy/data_validation.py	96.15%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #290      +/-   ##
==========================================
+ Coverage   76.50%   77.06%   +0.56%     
==========================================
  Files          20       21       +1     
  Lines        1345     1378      +33     
==========================================
+ Hits         1029     1062      +33     
  Misses        316      316              

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[jpreszler]

Sorry it took me so long to get to this. I've been starting work at 6am to overlap with coworkers in the UK and have been wiped out in the evenings.

I like splitting out the input validation, it's good progress to making things more streamlined.

There are a few suggestions I would make:

1. codecov has flagged a couple of things that don't have tests that would be easy to add but they seem fairly low value.
2. There's a lot of redundancy here. For example, there are 4 checks for _is_variable_dummy_coded with the only difference being the argument. Perhaps there could be a generic DataValidator class with methods for common checks that each experiment validator inherits from and the _input_validation methods just run the checks needed for that experiment.

I don't think either of these should hold up merging this PR. If you like the idea of 2, I could do that in the near future.

[drbenvincent]

Thanks @jpreszler. I've added tests for the scikit-learn versions of the experiments in test_input_validation.py