BUG/ENH: stats nan-policy with elementswise function

[josef-pkt]

Note from @mdhaber: This is a deep issue that is slated to be addressed as part of this project. Before composing a PR related to this issue, please comment below or - even better - consider resolving another open issue!

This is the general case for #8108
#8108 (comment)

currently there is no special handling of nans.

AFAICS, trimming functions trim1, trimboth treat nan as an extreme observation.

Reproducing code example:

>>> stats.zscore([1,1,1,1,1.1, 5, np.nan])
array([ nan,  nan,  nan,  nan,  nan,  nan,  nan])

>>> stats.sigmaclip([1,1,1,1,1.1, 5, np.nan])
<several RuntimeWarning: invalid value encountered in ...>
SigmaclipResult(clipped=array([], dtype=float64), lower=nan, upper=nan)

>>> stats.trimboth([1,1,1,1,1.1, 5, -5, np.nan], 0.25)
array([ 1. ,  1. ,  1. ,  1.1])