ICS-46 : Post aggregation reintroduced and general Grouping refactor


You can test the postAggregation function in the Pareto example which calculates a total value and then group percentages:

http://stavrakakis.github.io/insight/#/example/pareto

I also fixed a couple of bugs introduced into the bullet chart by a recent pull request
Why the need for the len variable here?  Is it an optimisation?