rethink Evaluator

[avibryant]

This makes a few related changes to Evaluator:

• it decouples it from Split and just has it directly operate on Iterable[T], which was the only part of a Split it ever cared about anyway.
• this makes it possible to apply an Evaluator to a tree or sub-tree, which could be useful in various ways in the future (eg if we want to change Splitter to produce candidate sub-trees instead of splits)
• in this context the Iterable[T] represents the leaves of the sub-tree, and so it's now labeled as such
• rather than using Infinity to represent an unacceptable level of error, it now returns Option[Double] so you can return None to signal that
• the sign has been reversed (ie this is now an error value not a "goodness" value)
• it doesn't return a new Split, just the error
• the root T is now also provided. None of the evaluators makes use of it, but there are at least two potential uses I have in mind:
  • considering a split unacceptable based on some ratio of the leaf T to the root T (eg, "all leaves must contain at least X% of the input Ys")
  • using more global optimization criteria (this comes up in particular in the "explanation tree" use case which I won't explain in more detail here)

This is WIP because I haven't fully updated the training code to make use of it.

[avibryant]

@tixxit ok I think this is ready now for review

[avibryant]

Got rid of the root part, and merged with #77 which should now be landed first.

[erik-stripe]

We should probably remove all the commented code in this PR once we agree it is really gone.