code: public Frustum setToOrtho (
          double left, double right, double bottom, double top, double near, double far) {
          return setToProjection(left, right, bottom, top, near, far, Vector3.UNIT_Z, true, false);
      }
docs: Sets this frustum to an orthographic one pointing in the Z- direction with the specified
  parameters determining its size (see the OpenGL documentation for <code>glOrtho</code>).
  
  @return a reference to this frustum, for chaining.