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.
being picky...but there's still a lot of logic here with the !hasValidDtgField and such...what about if we short circuit here and simplify the code:

```
def extractNewDTGFieldCandidate2(sft: SimpleFeatureType): Option[String] = {
    // if the attribute is actually present don't replace it
    if(index.getDtgDescriptor(sft).isDefined) return None

    // get all attributes which may be used
    val dtgCandidates = scanForTemporalAttributes(sft)

    // if there is a valid dtg candidate, then we can safely use it
    if (dtgCandidates.nonEmpty) {
      // if we are going to mutate UserData, notify the user
      emitDtgWarning(dtgCandidates)

      // we may wish to use the first acceptable attribute found, although we currently require just one match
      val firstDtgCandidate = dtgCandidates.headOption
      firstDtgCandidate.map { emitDtgNotification }
      firstDtgCandidate
    }
    else None
  }
```