Enhance Cube API to allow easier querying of leaves

[lbwexler]

Would like to be able to query all of the leaves that are consituents of a given cube row. Two options (we could do both):

1. Have some kind of a setting on the view that put the leaves on a bound store into a synthetic field e.g. "leaves?" . This would avoid the unwieldy dance we have to do if we want to keep a reference to them, but don't neccessarily want them to appear as nodes in the main view.

2. Have some kind of mechanism whereby we can easily use a drilldown key on a node to get the consituent leaves.

See the activity tracker in the admin tools for a common example of this pattern and the workaround we have to do.