Fix compilation with PGI for the development version of XIOS 2.0

 - Convert parse_xml.cpp to Fortran (port r573 to the trunk).
 - Modify date_interface to work-around a problem related to xios_duration type.
 - Reorganize the XIOS module to work-around a problem related to operator overloads for xios_duration and xios_date types.

git-svn-id: http://forge.ipsl.jussieu.fr/ioserver/svn/XIOS/branchs/xios-1.0@576 d613d254-d18a-4304-b296-5125d4c4d670