GEOMESA-590 Make ClosestAcceptableGeoHash more robust

* Function now returns an Option[GeoHash].
* Modulo check ensures returned GeoHashes are representable.
I see what you mean about the ugly `get`. This function should be deprecated soon however. For the time being, I'd suggest that we do a `foreach` over the `raster.mbgh` and issue the call to`registerRaster iff the mbgh is defined.