GEOMESA-144 Set temporal index variable by default if not selected by user

GEOMESA-144,GEOMESA-140
    
    GEOMESA-140 Warn users about missing temporal component of index
    This commit adds a warning when a user neglects to properly set SF_PROPERTY_START_TIME in the UserData. Such a mistake is non-fatal, but does impact performance.
    
    GEOMESA-144 Set temporal index variable by default if not selected by user
    This commit will set SF_PROPERTY_START_TIME for the user if only one time attribute is present in the SimpleFeatureType, and emit a notification to this effect.
Hmm... let's skip a return if we can.  Other options 1) breaking this into two functions and composing or 2) having a big if(!index.getDtgDescriptor(sft).isDefined) { ... } where the block calculates Some(value).

I think 1 might be best.