RegressionData dataclass

[lbluque]

Summary

• Implemented a v lightweight dataclass RegressionData to store regression model details in ClusterExpansion objects.
• Includes a convenience method to create from a scikit-learn or compatible object.
  This will greatly improve keeping track of provenance and details for reproducibility of cluster expansions.

Additional dependencies introduced (if any)

TODO (if any)

Checklist

• Code is in the standard Python style.
  Run pycodestyle and flake8
  on your local machine.
• Docstrings have been added in the Google docstring format.
  Run pydocstyle on your code.
• Tests have been added for any new functionality or bug fixes.
• All existing tests pass.