GEOMESA-557 Raster Index Schema

* Wrote RasterIndexSchema
* Wrote RasterIndexSchemaBuilder
* Wrote RasterIndexEntryEncoder
* Wrote RasterIndexEntryDecoder
* Moved raster code out of geomesa-core
* Updated comment to AccumuloRasterQueryPlanner
If we're using the lexigraphic encoding here, then we can lose the "ScientificNotation" part of the name, right?