Modal / nodal tracking

Right now, it's possible to feed modal data to things that expect nodal data and vice versa. Everything is just a bunch of numbers in a `DOFArray`. Especially if @thomasgibson's entropy-stable work manages to stick, this will be a persistent source of user error. We should perhaps have the `DOFArray` remember the [`discretization_key`](https://documen.tician.de/meshmode/discretization.html?highlight=discretization_key#meshmode.discretization.ElementGroupBase.discretization_key) for each group and check those on input to make sure they're as expected.