[Data description]

   - Interface cleanup.
     . gras_datadesc_by_name returns the searched type or NULL.
       That way, no variable is needed to use a type desc once, which makes
        the code clearer.
     . gras_datadesc_declare_[struct|union]_append_name is removed. The last
       two parameters were strings (field name, type name), leading to
       common errors.

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/simgrid/simgrid/trunk@97 48e7efb5-ca39-0410-a469-dd3cf9ba447f