Telephone line model monitoring

# What does it do?

This code controls a GUI window, which provides monitoring of the internal status of a telephone line modem. It shows, graphically:

    - the spectrum of the received signal.
    - the line model in use (when a known line model is being used).
    - the adapted coefficients of the canceller.

# How does it work?

This code uses the FLTK cross platform GUI toolkit. It works on X11 and Windows platforms. In addition to the basic FLTK toolkit, fltk_cartesian is also required.