Fix Flying Edges z-boundary condition with non-square input (Version 1.7)

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.

This MR applies the fix for #727 (closed) (as available in !2875 (merged)) to the version 1.7 release branch.