Fix: The conversion of a date to a number of seconds since the time origin was wrong.

Remove the work around in nc4_data_output.cpp which hid this bug.

Also backport this fix in XIOS 1.0.

git-svn-id: http://forge.ipsl.jussieu.fr/ioserver/svn/XIOS/branchs/xios-1.0@544 d613d254-d18a-4304-b296-5125d4c4d670