WADG should use overintegration

[inducer]

@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) Consider the current implementation:

grudge/grudge/op.py

Lines 645 to 653 in 2af3528

	group_data.append(
	# Based on https://arxiv.org/pdf/1608.03836.pdf
	# true_Minv ~ ref_Minv * ref_M * (1/jac_det) * ref_Minv
	actx.einsum("ei,ij,ej->ei",
	jac_inv,
	ref_mass_inverse,
	vec_i,
	tagged=(FirstAxisIsElementsTag(),))
	)

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

[inducer]

cc @MTCam

Newly relevant for quads.

[inducer]

Though in the long run we should probably switch to mass lumping, which is cheaper.

Made a separate issue for the mass-lumped inverse: #343.

cc @a-alveyblanc