Expose BufferedTiles as RDD numpy array

[echeipesh]

Having done map algebra to GeoTrellis layer, convert it to numpy RDD but include neighboring pixels so the python process can perform a python focal operation over tile boundaries.

Required information per record:

• Tile Key
• numpy array with all pixels
• bounds of central tile

This use case covers only the RDD[MultibandTile] -> RDD[BufferedTile[MultibandTile] -> PyRDD[numpy] conversion. We should avoid adding BufferedTile as a type to GeoPySpark tile/layer hierarchy. That type is not extensively supported in GeoTrellis and writing operations over it would likely run into a bunch of problems