Replace `MultiFunction`s with `DAGTraverser`s

[leo-collins]

DAGTraverser is the correct way (as it avoids UFL's hacky type system) to traverse a DAG, compared to the old MultiFunction and map_integrand_dag/map_expr_dag approach. There are a few instances of MultiFunction in firedrake currently and these should be replaced with DAGTraversers.

[1khazipov]

Hello! May I solve this issue?

[connorjward]

Please do! That would be fantastic.

Here's some information that should help:

• Our contribution guide: https://www.firedrakeproject.org/contribute
• An exemplar PR that does this for a specific MultiFunction: Use DAGTraverser instead of MultiFunction for ufl2unicode FEniCS/ufl#481
• A simple one to start with:

  firedrake/firedrake/mg/ufl_utils.py

  Line 25 in 6dbd8f0

  class CoarsenIntegrand(MultiFunction):

I'd recommend working on one MultiFunction at a time in separate pull requests.