Data aggregator doesn't have keys method

[richardjgowers]

Sim.selections.keys() # works
Sim.categories.keys() # works

# but
Sim.data.keys() # doesn't work
Sim.universes.keys() # doesn't work

If I'm using a dict like object, it makes sense to have a keys method for this. I had a quick look through the code, and it looks like the data aggregator is different, but ideally it should behave like the others? Maybe link keys to data._list?

[dotsdl]

Agreed. Originally I was wary of making aggregators look too much like dicts lest they be expected to behave like dicts in every way, since not every method available for dicts makes sense for these objects. I've since changed my mind, at least with respect to keys; the other aggregators reflect this, but I missed doing it for Data. I'll add a keys method to the Data aggregator.