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.
[Minor] sffiIterCfg having type Option[IteratorSetting] while stIdxIterCfg has type IteratorSetting seems somewhat inconsistent. Instead, moving the Some()  from this line into the definition of stIdxIterCfg may be more clear, even if stIdxIterCfg will never be None.