Improve CF compliance: Minor tweaks and improvements.

 - Bump the CF version to 1.5.
 - Use the same bounds dimension for all axis, including time axis.
 - Add a `standard_name` attribute for cell area variables.
 - Set the `interval` part of `cell_methods` attribute only when the write interval and the operation interval are different.

git-svn-id: http://forge.ipsl.jussieu.fr/ioserver/svn/XIOS/branchs/xios-1.0@614 d613d254-d18a-4304-b296-5125d4c4d670