Create a catenary curve between two points and a specified length.

Input parameters:

1. Pa → Start point for catenary curve.
2. Pb → End point for catenary curve.
3. Ln → Length of catenary curve. The length should be larger than the distance from the start point to the end point.
4. Gr → Direction of gravity.

Output parameters:

1. Ct → NURBS curve representing the catenary.
2. Ap → The apex of the catenary curve.