Homomorphism should error if there are multiple homomorphisms

[kris-brown]

Often there is a natural choice of a homomorphism, which we obtain by calling homomorphism(X,Y; kw...).

This is unambiguous when there is a unique one. It's a common footgun for users when there happen to be multiple ones, and the automated search returns a morphism which is not what we were thinking of. It's quite rare in fact to want a particular but arbitrary morphism (such that which one you get is dependent on the implementation details of homomorphism). This should be allowed behavior (via a keyword argument, unique) but not default.

This would technically be a breaking change, but we could also argue that code which (implicitly) relies on hom search implementation details is incorrect code.