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.
Disclaimer:  The ticket text says to convert this function to have an `Option` result-type, but despite that...  I'm wondering if we wouldn't rather use a default.  For example:

```scala
def getDtgFieldName(sft: SimpleFeatureType) = sft.getUserData.get(SF_PROPERTY_START_TIME) match {
  case value if value != null => value.toString
  case _ => SF_PROPERTY_START_TIME
```

With respect to errors, this moves the failure around somewhat, because if there is no date-time property set for the `SimpleFeatureType`, and if the user has NOT used the default field name, then the `SimpleFeature` will complain, but that shouldn't be much different than the case when we use an empty string as the date-time field name, should it?