Improve CF compliance: Various cosmetic changes.

 - Remove the `axis` attribute from the latitute and longitude coordinate variables when using curvilinear or unstructured grids.
 - Add the `axis="Z"` attribute only when the `positive` attribute is set for an axis.
 - Rename the `conventions` attribute to `Conventions`.
 - Add a global `title` attribute with the same content as the global `description` attribute. The old attribute might be removed in the future.
 - Remove the `title` attribute from the time coordinates.

git-svn-id: http://forge.ipsl.jussieu.fr/ioserver/svn/XIOS/branchs/xios-1.0@613 d613d254-d18a-4304-b296-5125d4c4d670