GEOMESA-238 Users should be able to use Spark on GeoMesa data

Added utility methods for initializing a Spark RDD on data
stored in GeoMesa.
I've been wondering if AccumuloDataStore should have a planQuery or explainQuery method on it.  I'd be for refactoring this up.  If you don't want to do it as part of this ticket, I can knock out what I'm thinking of as a separate ticket.