Migrate models along monomorphisms of theories

[epatters]

As a sequel to #374, it should be possible to push forward a model along a monomorphism of theories. Abstractly, this is the same thing as #374 but in practice it requires more work since

1. the theory map must be given explicitly instead of implictly, and
2. the migration can't be accomplished in simply by changing a pointer in the model but requires changing object and morphism types.