Groups use references to objects

[bgoli]

Currently Groups containing a simple list of objects that they collect. While this is an obvious way of doing things it can lead to the situation where, under certain circumstances a group membership can keep an object alive - even if deleted in the model.

I want to change the groups to use either a list of weakrefs or a weakrefdict instead and nullify the chances of creating reference cycles.

[bgoli]

First attempt committed as a0e1806