Allow fit methods to accept pd.Series and pd.DataFrame (#62)

[okiner-3]

close #62

Summary

• Allow fit methods of all estimators to accept pd.Series and pd.DataFrame in addition to np.ndarray
• Inputs are automatically converted to np.ndarray via to_numpy() inside the fit method
• Any object that implements to_numpy() (e.g. polars) is also supported

Changes

• Add _convert_to_ndarray helper in dte_adj/util.py
• Apply conversion in fit methods of all 6 estimator classes:
  • SimpleDistributionEstimator
  • AdjustedDistributionEstimator
  • SimpleStratifiedDistributionEstimator
  • AdjustedStratifiedDistributionEstimator
  • SimpleLocalDistributionEstimator
  • AdjustedLocalDistributionEstimator
• Add tests/test_pandas_input.py with 8 test cases