Simplify construction of Python `Consortium` object

[eddyashton]

The usual - this isn't what I started today doing, but it became a stumbling block for something else so seemed worth cleaning up.

This is an instance where we pass some very custom objects for very specific behaviour a few layers down, even though we have exactly 2 calling points! Scrap that - lots of people want to add Members after a Consortium has been constructed, initial members aren't special. There's still a little bit of threading in-and-out of a Consortium object for access to default values and script paths etc, but at least it's named now?

Let's see what breaks.