stats: sigma_clip: bug in definition of standard deviation

In the docstring to stats.sigma_clip is says `Defaults to the standard deviation (numpy.var)`. That is an error: `np.var` is the variance, not the standard deviation (that would be `np.std`).
For deviations close to 1 that is not an issue in practice, since sqrt(var) = std, but particularly for smaller numbers `np.var` will not result in a useful clipping.

Also, the `np.ma.xxx` version should be used, just to make sure it works if data is already a masked array.
If there is agreement that this is a bug to be fixed, I will add changelog and tests in a separate commit.