Form labeling?

[rckirby]

Firedrake, through FML, supports form labeling. It seems there are two interesting use cases in Irksome:

• tagging particular terms to split off explicitly in additively partitioned/IMEX methods. Making form labeling work here might simplify the interface so that users tag a piece of F rather than provide the explicit part through a keyword argument
• labeling in Galerkin (CG or DG) methods would allow us to use different quadrature rules for different terms, which could be of use in Andrews/Farrell structure preservation.