match_args_return: changed from a class to a function
match_args_return: changed from a class to a function

The problem with the class was that the decorator name was
showing up in gsw.__dict__ instead of the function name, and
this interferes with attempts to automatically generate the
tests.  With the function form of the decorator, the decorated
function is still a function, and shows up with its correct
name.

TEOS-10/python-gsw,castelao/python-gsw,rabernat/python-gsw,ocefpaf/python-gsw