more improvements to multiclass classification

This will error out unless clasS_probs is specified:

scala> "".split(",").map{s => val p = s.split(":"); (p(0), p(1))}.toMap
java.lang.ArrayIndexOutOfBoundsException: 1

A nicer solution is:

args.optional("class_probs").map{a => a.split(",")....(your fn that makes it into a map)}.getOrElse(Map[String, Double]())