Gets a 3D variant of the curve (z-coordinate equal to zero). 

The user of this method should pass a 3D curve to the method that corresponds to the type of 2D curve, otherwise an eNotThatKindOfClass exception will be thrown. Examples of corresponding curve types:

• OdGeRay2d - OdGeRay3d;
• OdGeLine2d - OdGeLine3d;
• OdGeLineSeg2d - OdGeLineSeg3d;
• OdGeCircArc2d - OdGeCircArc3d;
• OdGeEllipArc2d - OdGeEllipArc3d;
• OdGePolyline2d - OdGePolyline3d;
• OdGeNurbCurve2d - OdGeNurbCurve3d;