Laplace operators and mixed derivatives

[loeiten]

The Laplace operators currently uses the D2DXDY operators if mesh->g12 != 0 (and thereby also Laplace_perp), which currently returns 0 because the suggested stencil needs information from the corner points.

As far as I can see, one cannot make a stencil without using at least one corner cell when using mixed derivatives, so changing the stencil won't solve the problem.

I do not know whether or not this is a bug or not, but it should probably be written somewhere that one have to assume that DDY is small if mesh->g12 != 0 when using the Laplace and Laplace_perpoperators.

[johnomotani]

Partially (?) fixed by #1097, #1661. Could be properly fixed by making use of corner cells after changes in #1885.