GEOMESA-134: DTG field treatment in package object 'index' not safe

Wrap dtg variables in core.index in Options, since the temporal field in...

Also modify FilterToAccumulo to handle these changes.
Alright. Bottom line, if someone doesn't set the time index attribute properly on ingest, the index value has a null time component.  The index key however contains the date of ingest. This sounds Very Dangerous, and in the best of situations, time queries will be slower than they should be. 

Since the time indexing is optional, we should think about throwing a (very obvious) warning when the dtg name doesn't point to an existing attribute on ingest. Maybe a less obvious warning on queries too?