Changes
- Make `'ID'`and `'TYPE'` columns `pd.Categorical` instead of `str`, to reduce the memory spike when using `pd.pivot_table` in `sam_format_to_wide`.
- Added parameter in `QuantileRegressor` to use HiGHS solver, as recommended in <https://docs.scipy.org/doc/scipy/reference/optimize.linprog-highs.html>. This will also keep the package compatible with future versions of SciPy.