GEOMESA-557 Raster Index Schema

* Wrote RasterIndexSchema
* Wrote RasterIndexSchemaBuilder
* Wrote RasterIndexEntryEncoder
* Wrote RasterIndexEntryDecoder
* Moved raster code out of geomesa-core
* Updated comment to AccumuloRasterQueryPlanner
Right now, the schema is hard wired (which is the first step and just what we need at this point). Since the argument is not used, I'd much prefer a empty string to be used here.