*Features*
- Added Xbar and S chart utility `plot_xbs()`
- Added `fit_polyridge()` to return fitted model based on polynomial ridge regression. Useful for cross-validation of polyridge hyperparameters, and another metamodel option!
*Documentation*
- Significant improvements to transform documentation, including `tran_rename`, `tran_mutate`, `tran_filter`, `tran_select`, `tran_sample`, `tran_summarize`, and associated helper functions
*Fixes*
- Removed a use of `np.int`