'Get 3D world limits.'

def get_w_lims(self):

    (minx, maxx) = self.get_xlim3d()
    (miny, maxy) = self.get_ylim3d()
    (minz, maxz) = self.get_zlim3d()
    return (minx, maxx, miny, maxy, minz, maxz)
