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.
Where would the SimpleFeature complain? 

If we go with your solution, we'd still get a null Pointer Exception if the user includes a temporal or BETWEEN clause in the query after failing to set date-time property, since getDtgDescriptor will still return null. 
And in theory, someone could use BETWEEN for other attributes, correct?

Alternatively, we could change my version of FilterToAccumulo to use your suggestion and this:  
``` scala
val dtgFieldName  = index.getDtgFieldName
````
with the same behavior: no null Pointer, but slower execution on temporal queries. 

In my mind, there are two issues here: one is that we have two scala functions operating on an optional object, both of which may generate a null pointer exception. The other is that if someone has no temporal attribute and uses BETWEEN in their query, that will generate a null pointer exception when it should not. 

Now of course, we should also think about the situation when someone has not set SF_PROPERTY_START_TIME properly...but should have. Let me think....