Fix Flying Edges z-boundary condition with non-square input

There was a bug with Flying Edges checking the boundary conditions in the z-direction. It was comparing the z index to the size of the y dimension, not the z dimension. This simple typo was probably missed because most of the tests use square meshes. To catch the problem, you need a mesh that has a different number of points in the y and z directions and the contour has to go past the positive z boundary.

When this error was hit, the indices for the edges on that boundary were not recorded, and later interpolation used garbage values for edge's point ids.

Fixes #727 (closed)

Backport: release
Backport: release-1.8