#458 Grid Generator creates non-zero z values

[Tim Ayres]

The grid generator filter does not create zero values for z as expected; some values are small but non zero, e.g. 0.000001.

As a workaround, after creating the grid you can run a geometric function with "z = rint(z)" to round these values down to zero.

[Tim Ayres]

This was tested on MeshLab version 1.3.4Beta 64 bit on Windows 8.1.