Added functionality to calculate user-configurable metrics defined in python scripts.
Forgot to include the new python file in the previous commit.
Added checks to see if the metric should be computed based on the availability of the tissue model and the needle transform.
Merge branch 'master' of https://github.com/mholden8/PerkEvaluator
Only send current transform and time - metrics are responsible for storing previous values.
Merge pull request #13 from mholden8/master

Added user-configurable metrics.