mesh3d: handle messy data

[alexcjohnson]

Currently mesh3d does an in-place modification of the input data during supplyDefaults, noticed in the course of #2629. We should not be modifying the user data, rather making a new cleaned array, and we shouldn't be doing any of this in supplyDefaults either. Also in other places (like sankey) we reject non-integers rather than truncating, we should do that here too. This probably indicates there is other strange behavior there with messy data as well.

[etpinard]

cc @archmoj who discovered some of the same oddities during his isosurface work.