Analytic functions with blackbody

See #3077 
Yes, that test should work too (assuming it passes `u.spectral_energy(in_x)`), but still rewrite the test so that `Column` with unit unset goes through and do use `u.Quantity(in_x, u.AA)`, as it is substantially faster:
```
    if getattr(in_x, 'unit', None) is None:
        in_x = u.Quantity(in_x, u.AA)

In [6]: a = np.arange(100.)

In [7]: %timeit a * u.AA
10000 loops, best of 3: 42 µs per loop

In [8]: %timeit u.Quantity(a, u.AA)
100000 loops, best of 3: 7.34 µs per loop
```