WADG should use overintegration

@thomasgibson pointed out that our WADG could/should be able to take advantage of overintegration, which it currently doesn't. Consider the matrix we're supposed to apply:


(From [the paper](https://arxiv.org/pdf/1608.03836.pdf#page=6)) Consider the current implementation:

https://github.com/inducer/grudge/blob/2af3528391bf4cb07f5aacf43f7c6afcc18f693a/grudge/op.py#L645-L653

The `ref_Minv * ref_M * (1/jac_det) * ref_Minv * u` shold really be (in weird pseudocode):

```
ref_Minv * ref_M("overintegrated") * (1/project("overintegrated", jac_det) * project("overintegrated", ref_Minv * u)
```

cc @lukeolson