Add basic GeoDataset implementation

[adamjstewart]

This is a very primitive implementation of GeoDataset. It also adds datasets for Landsat, Sentinel, and CDL.

I've moved existing GeoDatasets to VisionDataset for now until the API for GeoDataset settles down.

Closes #10
Closes #11

[adamjstewart]

Next step is to write a Jupyter Notebook to demo the new GeoDataset. I'm thinking of creating datasets for Landsat and CDL, combining them into a ZipDataset, displaying some images, then training a simple model (maybe use pre-trained ResNet and only RGB bands) just to show how easy it is to use.