Warn when interpolation is taking place in vis

[inducer]

(Especially/only) if orders match, because then it might be unexpected.

cc @ShawnL00

[alexfikl]

Is the existing warning not sufficient? What other cases should it warn about?

meshmode/meshmode/discretization/visualization.py

Lines 1322 to 1325 in b15327f

	warn("Visualization discretization is identical to base discretization. "
	"To avoid the creation of a separate discretization for "
	"visualization, avoid passing vis_order unless needed.",
	stacklevel=2)

[inducer]

No, not in this case. This code:

pre_density_discr = Discretization(
    actx, mesh, InterpolatoryQuadratureGroupFactory(target_order)
    )

vis = make_visualizer(actx, pre_density_discr, target_order)

makes pre-density as GL and vis as GLL, I think. The warning didn't fire.

[alexfikl]

No, not in this case. This code:
makes pre-density as GL and vis as GLL, I think. The warning didn't fire.

Ah, yeah, it wouldn't fire in that case. I'm not sure how I feel about making that a warning though.. all the visualizers in pytential will complain :(

Definitely could use better documentation for when it interpolates and to what elements.