Interface for LGL mass lumping?

See #344 for context. Diagonal mass matrices on LGL points are a common approximation. I'm thinking they should not be the default behavior, since they're not exact, and exact evaluation is totally doable (if more expensive). (Do we agree on that?) Hence we need permission from the user to use lumping. How should that permission be given?

- Once for the whole `DiscretizationCollection`? (i.e. as a flag living on that)
- Once per (volume) element group? (and likely inherited by corresponding surface groups?)
- Per-operation?
- A combination of the above?

cc @MTCam @a-alveyblanc @alexfikl @lukeolson @majosm @anderson2981