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.
I'm concerned that in this case, a single method would be doing too much, since the case class does three things:
1) checks the SFT for a dtg attribute
2) finds a good candidate for a dtg attribute
3) warns the user that they are doing something wrong.

Also, since the UserData may need to be mutated, I'm prefer the mutation to be done in the  AccumuloDataStore, and for the reasons to be clear in that class as well.  

The alternative is to call a single method, with a return type of Option[String] and do this: 
``` scala
TemporalIndexCheck.dtgCandidate(sft).map { name => userData.put(core.index.SF_PROPERTY_START_TIME, name) }
```
but this seems intuitive.