Allow sizeThreshold == 1.0 in SMESH StdMeshersBuilder_Cartesian_3D.SetGrid()

Hello,

In order to produce a purely hexahedral mesh I currently use the closest float value superior to 1.0 - in Python; numpy.nextafter(1.0, 2).

That’s a workaround, and I don’t understand why this mesher conceptually forbids pure hexa meshes by asserting sizeThreshold > 1.0.

Thank you !