add Collection.prototype.aggregate()

[bsouthga]

Currently, Tyranid collections don't seem to have an aggregate method, so col.db.aggregate() is necessary.

[dimensia]

What are you looking for col.aggregate() to do that is different from col.db.aggregate()?

[bsouthga]

potentially allow inclusion of computed / populated properties, though not sure if the complexity would be too high. Multiple passes of aggregation / moving the logic to javascript might be necessary.