GEOMESA-445 Create Temporal Density Iterator

depends... maybe IllegalStateException? I assume nothing should get this far that doesn't have a dtg field. If this is a normal use-case, then IllegalArgumentException or just a RuntimeException might be more appropriate. The main point is that it has a more useful error message than what you would get right now, which is not very descriptive:
java.util.NoSuchElementException: None.get
  at scala.None$.get(Option.scala:322)