GEOMESA-557 Raster Index Schema

* Wrote RasterIndexSchema
* Wrote RasterIndexSchemaBuilder
* Wrote RasterIndexEntryEncoder
* Wrote RasterIndexEntryDecoder
* Moved raster code out of geomesa-core
* Updated comment to AccumuloRasterQueryPlanner
The index schema should be passed as the argument to `AccumuloRasterQueryPlanner`. That will permit its use later .....