Check for and prevent initialization of NDData with Quantity

This PR simply raises an error if you try to initialize ``NDData`` or ``StdDevUncertainty`` with a ``Quantity``.

Once I know which release this is targeted for (if any--it may be completely the wrong way to go) I'll add a changelog entry.

I could, if desired, do something a little more sophisticated (and perhaps more sensible) than this, though I don't think what is below is a long-term solution:

+ Initializing ``NDData`` with a ``Quantity`` could set the ``NDData.data`` (as a ``numpy.ndarray``) and ``NDData.unit``
+ Initializing ``StdDevUncertainty`` with ``Quantity`` could convert that quantity to the units of ``StdDevUncertainty.parent_nddata`` then set ``StdDevUncertainty.array`` with the result, raising an error if ``parent_nddata is None``.