Work on ZonalSummarySpec
Add return type annotations to Zonal Summary Methods
Fix TiledPolygonalZonalSummarySpec to work with DataSource changes
Merge remote-tracking branch 'origin/ZonalSummaryFix' into foo
Merge remote-tracking branch 'origin/FixZonalSummarySpec' into foo

Conflicts:
	src/test/scala/geotrellis/raster/op/zonal/TiledPolygonalZonalSummarySpec.scala
Change ZonalMin, ZonalMax, ZonalMean to return Int
Merge branch 'ZonalSummaryFix' into foo