Refactor AV to use the diffusion operator

[MTCam]

Let me ask the elephant-in-the-room question here: Given that we have variable-coefficient diffusion, why do we want this? As @majosm can possibly attest, getting diffusion with elementwise-discontinuous coefficients right is not entirely trivial. And the already-implemented diffusion operator literally implements the same operator. It doesn't help that this operator is not well-tested.

I'd be OK with this becoming a deprecated wrapper of the diffusion operator.

Originally posted by @inducer in #582 (comment)