Add modelling notebooks

[Peter9192]

This PR adds two notebooks on modelling:

• An general overview of using pycaret with different (combinations of) models
• A tutorial on adding your own sklearn-compatible estimator, specifically, a pyphenology-like model class.

Also changed the split_time utility method for eobs.

Closes #200 Refs #198 by fixing xarray (temporary patch) Refs #194 through notebook on adding models