- Group By filters
  - groupByFlat : do a groupBy but collapse it into a flat list
  - flattenGroupBy : flatten an existing groupBy list
  - itemGroup : a groupBy object that has the following structure
  	- key : value of the current group by target
	- values : list of items that have the property for the target
	  	with the same value as the 'key' value mentioned above
    - prototype._targets : list of targets the current groupb item is the root of
	- prototype._targetValues : the values of the _targets in the previous list. This is set only for itemGroups that are result of flatttening a groupBy or groupByFlat
	- prototype._getTargets, prototype._setTargets : accessor methods for the '_targets' property on the prototype
	- prototype._getTargetValues, prototype._setTargetValues : accessor methods for the '_targets' values on the prototype