code: public Vector3 z(Vector3 q) {
  		float z = z_value(q);
  		return new Vector3(q.x, q.y, z);
  	}
docs: compute the Z value for the X,Y values of q.
  assume this triangle represent a plane --> q does NOT need to be contained
  in this triangle.
  
  @param q query point (its Z value is ignored).
  @return q with updated Z value.