Rename `marginal` attribute of MarginalModel

[jrising]

The attributes of the MarginalModel object are confusingly (and incorrectly) named. The marginal attribute does not contain itself some kind of marginal model object, or anything like it. Rather, it contains a perturbed model. I propose that the marginal attribute be renamed perturbed.

[davidanthoff]

Another option might be to rename the fields to model1 and model2.

[corakingdon]

or base and modified.

[davidanthoff]

@jrising do you have a preference for one of these options? We are all indifferent here, but happy to change it.

[jrising]

I think that base and modified are sensible, if we don't want to assume that a MarginalModel was created only by the SCC process. That would be my preference.

[lrennels]

This would be breaking, so we we should make a call here.

[davidanthoff]

Indeed. I thought we had made a decision at some point? I have to admit that I don't care that much. But, whatever we decide now will stick :)