Morphisms between atttribute types in acset schema

The category of attributes in the schema is assumed to be discrete, but it would helpful to allow the specification of morphisms.

For example, the schema for a free diagram is currently as defined:

```julia
@present TheoryFreeDiagram <: TheoryGraph begin
Ob::AttrType
Hom::AttrType
ob::Attr(V,Ob)
hom::Attr(E,Hom)
end
```

Since a free diagram is supposed to be a functor out of a freely generated category, it would be better to have:

```julia
@present TheoryFreeDiagram <: TheoryGraph begin
Ob::AttrType
Hom::AttrType
dom::AttrHom(Hom,Ob)
codom::AttrHom(Hom,Ob)

ob::Attr(V,Ob)
hom::Attr(E,Hom)

compose(src, ob) == compose(hom, dom)
compose(tgt, ob) == compose(hom, codom)
end
```

The intended interpretation is that whatever Julia types instantiate `Ob` and `Hom` should have methods `dom(::Hom)::Ob` and `codom(::Hom)::Ob` and then the instance data should satisfy the two equations.

At least to start with, this feature would be for documentation/specification purposes and would not provide any new functionality. Thoughts on this, @olynch?