Raster preview in viewport for Raster components

[andrewheumann]

This builds on PR #19 (So I'd wait to review this one until that one is merged)

• Cleans up GetTileRange to use more descriptive types rather that lists of lists of ints
• Creates a new HeronRasterPreviewComponent abstract class with raster preview functionality, and makes RestRaster and SlippyRaster inherit from this
• Does a little cleanup of unnecessary namespaces / references (e.g. GH_IO.Serialization.GH_IWriter => GH_IO.Serialization.GH_IWriter)