[WIP] Add Polygon model support

[Om-Doiphode]

Fixes issue #758

1. Add MaskRCNN to deepforest/models/ directory.
2. Create a polygon dataset class.
3. Confirm that preprocess.split_raster and other utilities/visualization functions can take in polygon predictions.
4. Create a example ipython notebook showing how to train a polygon model, train an initial model using open source polygon data