Create a set of equally spaced surface contours.

Input parameters:

1. Sf → Surface to contour.
2. Pt → Contour base point
3. Nr → Contour slice normal direction.
4. Sp → Spacing between contour layers.

Output parameters:

1. Ct → Contour curves, sorted by layer
2. Ly → Layer, i.e. contour section offset, given as an integer. Layer zero is the section through the base point.