GeoTrellis ArrowTile

[echeipesh]

Subclass geotrellis.raster.Tile interface where cells are somehow stored in arrow record.

Objectives are:

• provide a concrete class for TileUDT to use
• see how much of the interface can be satisfied
• produce micro-benchmarks vs ArrayTile

[jpolchlo]

I think this is covered, at least preliminarily, by #17