Improve CF compliance: Write the "cell_methods" metadata.

Also try to use the time units defined by the UDUnits specification.

git-svn-id: http://forge.ipsl.jussieu.fr/ioserver/svn/XIOS/branchs/xios-1.0@612 d613d254-d18a-4304-b296-5125d4c4d670