Pretty-print equations in models of discrete theories

[KevinDCarlson]

This pretty-prints equations in discrete double models for the first time. An example is being tested in the snapshot test test_equality.dbltt, which exhibits output looking like so:

generate CommutativeSquare
#/ result: model generated by 4 objects and 4 morphisms
#/ NW : Entity
#/ NE : Entity
#/ SW : Entity
#/ SE : Entity
#/ t : NW -> NE : Hom Entity
#/ l : NW -> SW : Hom Entity
#/ r : NE -> SE : Hom Entity
#/ b : SW -> SE : Hom Entity
#/ t ⋅ r = l ⋅ b : Hom Entity(NW,SE)

Arguably the suffix could be any of

• : NW -> SE
• (Hom Entity)(NW,SE)
• Hom Entity NW SE, or if we modify the morphism type printing, `
• Hom(Entity)(NW,SE).

I think the last option is probably my favorite.

(EDIT: commit added to pick the last option.)

[epatters]

Thanks! I've cleaned this up by upgrading FgDblModel to FpDblModel and adding a method equation to that trait.