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.
That doesn't quite work, but this 
```` scala
sft.getAttributeDescriptors.asScala.toList
     .withFilter{_.getType.getBinding == classOf[java.util.Date] }
     .map {_.getLocalName}
```
does, since I'm using JavaConverters. Change made.