Add "where" based ufunc masked array support decorator

[DocOtak]

This PR replaces the match_args_return decorator in _wrapped_ufuncs with the one proposed/discussed in #97 (comment) .

I don't think it's ready for merging: needs tests, more discussion, and the non ufunc functions need to be considered. But I wanted to show what a change like this might look like.

[efiring]

This certainly looks more elegant than what I originally came up with. I'm entirely open
to making the switch once it's clear it won't break anything.

[ocefpaf]

@DocOtak and @efiring I believe that we should mint a new release with the latest changes and fixes. Should we wait to include this PR in it or should we release it now and mint a fresh one when this is done?

[efiring]

I think it makes sense to release both GSW-C and GSW-Python now.

[DocOtak]

I agree, don't wait on this PR.

[ocefpaf]

I think it makes sense to release both GSW-C and GSW-Python now.

Done and done!

[DocOtak]

I am interested in continuing work on this, but would appreciate some guidance. This decorator does not work with the non ufunc functions we have.

Reading through my code comments, I think I should at least make it clear that the where=True ufunc default is the default of all ufuncs by numpy definition, not just here in this decorator. i.e. if you don't pass a where arg, numpy passes True for you.