WPS process to rank points along a track/route

Update of ktcorby's efforts on PR#73
Caused by: java.lang.UnsupportedOperationException: empty.maxBy
	at scala.collection.TraversableOnce$class.maxBy(TraversableOnce.scala:223)
	at scala.collection.AbstractTraversable.maxBy(Traversable.scala:105)
	at org.locationtech.geomesa.core.process.rank.ResultBean$.fromRankingValues(ResultBean.scala:149)