def METHOD_NAME( self, dir_path) :
    """
    Check whether the directory exists, and if not create it.
    Parameter
    ---------
    dir_path : string
       Relative path from the directory where the diagnostics
       are written
    """
    # The following operations are done only by the first processor.
    if self.rank == 0 :
        # Get the full path
        full_path = os.path.join( self.write_dir, dir_path )
        # Check wether it exists, and create it if needed
        if os.path.exists(full_path) == False :
            try:
                os.makedirs(full_path)
            except OSError :
                pass