Roadmap: GAT of GATs

@epatters @mehalter @bosonbaas, here are some tasks I think we should address for the next push on Catlab (aka Gatlab).

- [ ] CT constructions
- [ ] Product/Sum
- [ ] Span/Cospan
- [ ] Functor/Instance
- [ ] Category of Elements
- [ ] GAT Functors, injective, surjective to talk about relationships between algebraic theories
- [ ] Causal Theories
- [ ] Statistical Theories
- [ ] Physical Theories
- [ ] Compute pushout/pullback of GAT functors
- [ ] Cospan category for open GATs
- [ ] Pushout/pullback within a Cat, can we infer the rules for composition of FinSet cospans from the definition of FinSet morphisms?
- [ ] Relation orient computing: functors from an arbitrary GAT into Rel
- [ ] Generic Operads
- [ ] Syntax
- [ ] Drawing
- [ ] Substitution based on term matching
- [ ] Axiom Support
- [ ] Generating term rewriting rules from the specification of the axioms of a GAT