'Test subok can be used to keep class, or to insist on Quantity'

def test_subok(self):

    class MyQuantitySubclass(u.Quantity, ):
        pass
    myq = MyQuantitySubclass(np.arange(10.0), u.m)
    assert (type(u.Quantity(myq)) is u.Quantity)
    assert (type(u.Quantity(myq, subok=True)) is MyQuantitySubclass)
    assert (type(u.Quantity(myq, u.km)) is u.Quantity)
    assert (type(u.Quantity(myq, u.km, subok=True)) is MyQuantitySubclass)
