DEP: Proposal to deprecate the msort convenience function

[rossbar]

This is a proposal to deprecate the msort function, which is currently exposed in the main numpy namespace. The function is a convenience function for performing a sort along the 0-th axis:

numpy/numpy/lib/function_base.py

Lines 3677 to 3679 in 7776bc6

	b = array(a, subok=True, copy=True)
	b.sort(0)
	return b

which can be accomplished more idiomatically with np.sort(a, axis=0).

Re: impact assessment per NEP 23 - I have grepped through other major scientific Python libraries (scipy, matplotlib, scikit-image, scikit-learn, networkx) and there were no instances of msort being used. msort is found in astropy in the units subpackage in a listing of subclass-safe functions imported from numpy; though it does not appear to actually be used within astropy and should be straightforward to remove.

[mattip]

one small nit about formatting the warning, otherwise LGTM

[mattip]

Thanks @rossbar