Build utility function for getting tracked data

[Itangalo]

When building statistics it would be useful to have something like getTrackedData('VP') that gives an array of the tracked VP data for agents. Then functions like getAverage(vpData, 'increaseSum') could be used.

[Itangalo]

It wouldn't be a method on agents – this should collect data from all agents.

[Itangalo]

Also! The getTrackedData() function should set undefined values to 0. That would help.