WIP: Added TileIdPartitioner to Context

1. Added TileIdPartitioner to Context - We need it to save it to disk when calling RasterRDD.save
Also, downstream operations from RasterRDD need it to reason about how the RDD is partitioned
2. Added a helper utility RasterStorageStats to various block/tile/partition-level stats from an on-disk raster
3. Removed unused code from SaveRasterFunctions