Selections aggregator should now give AtomGroups.
Selections aggregator should now give AtomGroups.

The Selections aggregator should generate an AtomGroup from the stored
selection each time it is called. Also, added a setitem method for
convenience (works same as add.selection()).

andreabedini/datreant,datreant/datreant,datreant/datreant.data,datreant/datreant,datreant/datreant.core,datreant/datreant.core,dotsdl/datreant