[CF-metadata] axis attribute

[Jonathan Gregory]

Dear Bert, John and all

I think the usefulness of the axis attribute (for X and Y) depends on being
able to know which are horizontal coordinates without having to identify them
by standard name or units. Even if that's only for plotting, it could be
useful. However, the attribute is optional of course.

My original issue was whether we should (a) broaden it to any horizontal 1D
coord i.e. allow it for rotated pole lat-lon or (b) restrict it to genuine lat
and lon. I now vote for (a) although I initially favoured (b).

Cheers

Jonathan