More features for migration action on morphisms

1. As mentioned in the review of https://github.com/AlgebraicJulia/Catlab.jl/pull/710 - it would be nice to have methods for migrating morphisms that presume the dom and codom have already been migrated (to avoid duplicating work when migrating a whole diagram, for example).
2. Other kinds of data migrations (e.g. sigma) should act on morphisms, too.