BUG: Define <, <=, >, >= for masked arrays

[seberg]

This defines the comparison operators (other than == and !=)
explicitly for masked arrays.
The mask is ignored for the resuling res._data (unlike == and
!= which take the mask into account.

Closes gh-21770, although the way that masked arrays propagate the
fill-value seems generally broken and error prone.

[seberg]

Arg, no... this should not really be necessary, closing for now.

[mhvk]

Looks good! I agree with the actual underlying data values just being the result of the comparison.

[mhvk]

Oops, I hadn't seen you closed it! Why would this not be necessary? It is true that in principle this could be done inside __array_ufunc__, but that would be a bigger effort... Or is it just that this is more than is needed to fix #14727?

[seberg]

Yeah, I will reopen (and remove the release notes, I don't think they are actually necessary).

[seberg]

I guess my point was that the fill-value propagation seems broken for all ufuncs at least when the result dtype does not match the (first) input. The best solution is probably indeed __array_ufunc__ though (maybe not the only solution, but presumably much cleaner than the alternative).

EDIT: Maybe unclear... This is sufficient to fix all the issues. However, if ufuncs were not broken, this should not be necessary. For one, it leaves np.logical_or broken the same way.

[mhvk]

Yes, agreed that fill-value propagation is broken/tricky. But separate from this PR at least.

[charris]

@seberg Are you happy with this?

[seberg]

Yes, happy enough andnd it should fix (most) of astropy's current issue. We still need to figure out if there is something to do about gh-21784 or not (but it is distinct).

[charris]

Thanks Sebastian.