GEOMESA-557 Raster Index Schema

* Wrote RasterIndexSchema
* Wrote RasterIndexSchemaBuilder
* Wrote RasterIndexEntryEncoder
* Wrote RasterIndexEntryDecoder
* Moved raster code out of geomesa-core
* Updated comment to AccumuloRasterQueryPlanner
To follow up from a previous comment, using the index schema here is desired. Instead of the TODO, actually wire things up.