`setleafcontext(::Model, ::AbstractContext)`

[penelopeysm]

Most usecases for contextualize are to change the leaf context, i.e., there's often a pattern that looks like this:

new_ctx = setleafcontext(model.context, new_leaf)
new_model = contextualize(model, new_ctx)

Probably easiest to define setleafcontext(model, new_leaf) that will do this.

[yebai]

Do we still need this if we eventually eliminate parent contexts?

[penelopeysm]

Nope, in that case just contextualize would be enough.

[penelopeysm]

(To be fair, in that case the entire setleafcontext function would not exist at all, not just this method)

[penelopeysm]

closed #1057