Convert parse_xml.cpp to Fortran to ease compiling with PGI compilers.

Previously linking would fail because a special compiler switch is required when linking a CPP code with a Fortran library using PGI. To avoid having to handle that special case, switch to Fortran.

git-svn-id: http://forge.ipsl.jussieu.fr/ioserver/svn/XIOS/branchs/xios-1.0@573 d613d254-d18a-4304-b296-5125d4c4d670