CF compliance: Fix `cell_methods` attribute.

The specifications do not allow using an auxiliary coordinate to identify an axis. Instead always use the generic standard name "time".

git-svn-id: http://forge.ipsl.jussieu.fr/ioserver/svn/XIOS/branchs/xios-1.0@618 d613d254-d18a-4304-b296-5125d4c4d670