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

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

```julia
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.