[CF-metadata] coordinates attribute

[Jonathan Gregory]

Dear Juerg

The grid_mapping attribute *is* attached to the data variable, because the
grid mapping is a property of the data variable and not of any of its
individual coordinate variables. The coordinates attribute is the same. It
provides auxiliary coordinates for the data variable. Some of the auxiliary
coordinates are multidimensional (2D lat and lon being the commonest example).
Such auxiliary coordinates clearly do not belong to any particular coordinate
variable. They belong to the data variable. Hence I think the current draft
is correct to have the coordinates attribute as an attribute of the
data variable.

Best wishes

Jonathan