add_demo — Add an entry in the demos list
add_demo(title,path)
a character string, the demo title
a character string, the path of the scilab script associated with the demo
    This function adds a new entry in the demos list. The demo should be
    executed by a Scilab script file. If the given title already exists
    in the demo list associated with the same file nothing is done.
    The function checks if the file exist.