Add uber lyft dask xgboost example

[ncclementi]

This is an adapted notebook with more context based on the dask-xgboost repo one. It is the notebook used for this examples doc https://docs.coiled.io/user_guide/examples/dask-xgboost.html, it should be a good starting point.

[mrocklin]

Pushed some changes. This is mostly basic stuff to try to streamline the notebook.