NaN matcher for np.nan

[danodonovan]

How would you feel about a PR for a matcher that checked if a value was NaN (for the numpy definition of NaN)?

Perhaps something like is_nan() ? (and using np.isnan under the hood).

NB By design NaN != NaN so equal_to(np.nan) wouldn't work here.

[mwilliamson]

Hmm, anything that relies on numpy should probably go in a separate module. I'm not opposed in principle though.

The alternative would be to add a more general is_ matcher, so you can write is_(np.nan).