Fix bug with individual level label

[jwreep]

Fixes #434

I figured it was probably better to simply use np.atleast_1d to solve this problem, rather than introducing if/else logic.

>>> import astropy.units as u
>>> import fiasco
>>> s_10 = fiasco.Ion('S X', 1*u.MK)
>>> s_10[0].label
array(['2s2 2p3 4S3/2'], dtype='<U13')
>>> s_10[:1].label
array(['2s2 2p3 4S3/2'], dtype='<U13')

Do we want to add a test as well?

[wtbarnes]

A much better solution than my original suggestion. Thanks! A test would be great. The code snippet in your comment would be a sufficient test.

[wtbarnes]

Thanks Jeff!