function err(param) {
            throw Error("Missing parameter: " + param);
          }
@class A geometric definition of a cone (and cylinder as a special case) with
support for mesh creation/representation. The class is currently still
incomplete in that it doesn't provide any other features than the
construction of a cone shaped mesh.
@augments toxi.Vec3D
@member toxi
@param pos
           centre position
@param dir
           direction vector
@param rNorth
           radius on the side in the forward direction
@param rSouth
           radius on the side in the opposite direction
@param len
           length of the cone