Create a catenary curve between two end points and an extremum point.

Input parameters:

1. Pa → Start point for catenary curve.
2. Pb → End point for catenary curve.
3. Ex → Extremum point which sets the maximum/minimum of the catenary curve.
4. Gr → Direction of gravity.

Output parameters:

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