weak_local_grad: Computes rst derivatives too many times

[inducer]

This code:

grudge/grudge/op.py

Lines 354 to 359 in 1c44c4b

	return make_obj_array(
	[_apply_stiffness_transpose_operator(dcoll,
	dof_desc.DD_VOLUME,
	dd, vec, xyz_axis)
	for xyz_axis in range(dcoll.dim)]
	)

computes the r,s,t (i.e. reference) derivatives once per dimension. We can compute them once and reuse them. In 3D, that saves a factor of 3 in work.

This is something I should have caught during review of #74.

cc @thomasgibson @lukeolson

[inducer]

I think I was mistaken here. For general curvilinear elements, the redundant computation can't be avoided, as the inverse metric derivatives can vary. The optimization I talk about here is only possible in the affine setting, cf. #161.