Diagram morphism search

[kris-brown]

Diagram morphisms are important in some other projects (e.g. ModelExploration, Decapodes) yet are incredibly tedious to specify manually.

There are many related search problems and it would be nice to see them done in a unified way, but for now they are done piecemeal:

• Enumerating Hom(::ACSet,::ACSet)
• Enumerating Hom(::FinFunctor,::FinFunctor)
• Enumerating Hom(::FinCat,::Fincat)
• Enumerating Hom(::Diagram{FinCat,ACSet},::Diagram{FinCat,ACSet})

Efficiency was not prioritized because FinCats generally have small data - the only one that must be efficient (and is efficient) is the first task.

Another to-do to make this useful is to be able to provide partial initial data to restrict the number of outputs.

Caveats:

• to do these enumerations, the (undecidable) word problem for categories must be solved. In general we might want to use MetaTheory for this, for now a makeshift solution is implemented. A more satisfying solution might allow the user to provide their own decision procedure on a per-category basis (which could be done automatically for large classes of categories).
• although the number of shape maps (i.e. FinFunctors) can be infinite, a keyword restricts the max # of cycles in composed generators