Modem performance monitoring

# What does it do?

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

    - the constellation, for PSK, QAM and other similar modulations.
    - the equalizer status, for modems with adaptive equalizers.
    - the carrier frequency.
    - the symbol timing correction.

# 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.