Added save action to ETL - #1764

    EtlConf(args) foreach { conf =>
      val etl = Etl(conf)

      val sourceTiles = etl.load[ProjectedExtent, Tile]
      val (zoom, tiled) = etl.tile[ProjectedExtent, Tile, SpatialKey](sourceTiles)

      etl.save[SpatialKey, Tile](LayerId(etl.input.name, zoom), tiled, { (attributeStore, layerWriter, layerId, rdd) =>
        // Save off histogram computed from zoom 9, store in zoom 0's attributes.
        if(layerId.zoom == 9) {
          val histogram = rdd.histogram(512)
          attributeStore.write(
            layerId.copy(zoom = 0),
            "histogram",
            histogram: Histogram[Double])
        }
      })
    }