Merge pull request #13 from mholden8/master

Added user-configurable metrics.
Fixed skipping first timestamp in analysis for Python metrics.
Re #12: Only calculate python metrics if the specified metrics directory had any metrics defined in it.
Re #12: Nodes are no longer searched for at every timestamp - each buffer will have an associated node (which doesn't change for its lifetime in the Perk Evaluator module).
Re #6: Changed default needle orientation for metric calculation to match needle orientation protocol. Moved needle orientation options to "Advanced" section.
Attempting to fix according to transform node API changes.