Add calculated/derived metrics support

[rossbayer]

Add ability to create a metric that derives from another metric or metrics. It should support a reactive model; for instance, if I am measuring estimated vs actual story points, a calculated metric could be story point variance. Any time the estimated or actual story points are calculated, the derived metric will be recalculated and persisted.

Allow user to specify a function that will take the original values as inputs, then can return a derived value to store.