Add Feature classes to geotrellis.feature

Other changes:
 - Rename [Point,Line,Polygon]Set to Multi[Point,Line,Polygon]
 - GeometryCollection now is a Geometry, and Geometry does not have methods that do not apply
 - Renamed the inner jts.Geometry to jtsGeometry. It differentiates it from the `geom` of the Feature. Might want to make it private at some point, but maybe not in case someone really wants to get to the jts geometry. 
 
TODO: Implicit conversions from Features, Seq of Features to Geometries so that functions taking in Geometries can take Features without the user having to do the conversion.