fix getindex methods for composites

[corakingdon]

We need to be able to access the parameters and variables that are explicitly available at the composite level, probably via the following:

m[:top_composite_name, :exposed_variable_name]

I think we definitely need that method working for v1.0.0, which currently errors.

We had also discussed the ability to use dot-syntax to go down the tree to access values that maybe aren't bubbled up? something like:

m.top_composite.leaf_component.variable_name

But maybe that's not necessary for v1.0.0

[corakingdon]

@lrennels I found this original issue for datum access methods, so I closed the other one

[lrennels]

#704