Annotating Decapodes with types

Currently, Decapodes are flexible. That is, an object of the type `SummationDecapode` could represent equations in the 2D Exterior Calculus, or 1D Exterior Calculus, and so on.

Programming with Decapodes would become more well-typed if the Decapode objects had something like a type parameter with the degree of the DEC being used. Compilation steps like type inference could then automatically pick the correct set of inference rules.

The typing system could most likely be improved in other ways, but some brainstorming needs to happen first.