Fix incorrect output values when using a field_ref or the @ operator on a...

Fix incorrect output values when using a field_ref or the @ operator on a field computed from an operation.

The computed values were never transmitted from the source field to the destination field.

git-svn-id: http://forge.ipsl.jussieu.fr/ioserver/svn/XIOS/branchs/xios-1.0@562 d613d254-d18a-4304-b296-5125d4c4d670