Remove the SinglebandRaster and MultibandRaster Types

[jbouffard]

During my refactor, I noticed two types which I had not seen before: SinglebandRaster and MultibandRaster. Looking into it further, I found that those two types represent Raster[Tile] and Raster[MultibandTile], respectively. However, these types are used a lot less than their de-sugared counterparts. Therefore, we might want to consider removing these types outright to simplify the API.