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.
Is this more readable?
```
sft.getAttributeDescriptors.asScala.toList
  .filter(_.getType.getBinding.getCanonicalName == "java.util.Date")
  .map(_.getLocalName.toString)
```

If the original is preferred, permute the line "theName = descriptor.getLocalName" after the if guard.