Add tool for interactively generating training data

[robbibt]

A thought came to mind which I believe would greatly increase the utility of the Sandbox for students, which is for students to be able to create their own training data in the notebooks. At present the students are creating and labelling polygons in ENVI or Arc, then converting to shp, exporting, and then importing into the Sandbox to train a classifier

This is something that is asked regularly, and would be a great addition to the repo. Several people within DEA have implement something like this in the past (e.g. @s-m-t-c @cbur24 @MatthewJA) - it'd be great to pool ideas and come up with a nice, simple tool for interactively generating training data based on DEA data.