Was going to say that it might be better a baseclass + wrap at the end (for matrix support, but matrix support is bad anyway...), but then the non-nan code does the same.  Which makes me wonder, would it be sensible to just create a `where=` kwarg instead making the nan-funcs just tiny wrappers? Of course I could dream about having where for usual ufunc.reduce, but I think it probably would require larger additions to the nditer.