Add TileFeature Type - #1429

In this pull request the TileFeature[T, D] type is added.

Points to Examine

• The TileFeature[T <: CellGrid,D: ? => BundleMethods[D]] type takes two type parameters, but the merge methods (in both raster and spark assume that T <: Tile. There are no assumptions on D other than it being a member of the bundle type class.
• There is a question of range and precision of the data D. Right now they are treated as integer entities. Double versions are possible as well, but since one of the supported types of D is Seq[Tile], there is a question of how (best) to handle the situation when the data are mixed integers and floats.
• I found that I had to add these secondary conversions from subclasses of BundleMethods[D] to BundleMethods[D] in order to get the constraints on TileFeatureMergeMethods to be satisfied. The issue seems to be that the Scala compiler does not look more than one layer deep when trying to satisfy these constraints. If there is a better way to handle this, I am open to it.

Still Need To

• Provide Bundle Methods for Seq[Tile]
• Provide Appropriate I/O support for TileFeatures
• Provide Merge Methods for RDDs of TileFeatures
• Move bundle methods to their own directory
• Write ScalaDocs
• Integrate Tests into PersistenceSpec