Telephone line model

# What does it do?

The telephone line modelling module provides simple modelling of one way and two way telephone lines.

The path being modelled is:

    -    terminal
    -      | < hybrid echo (2-way models)
    -      |
    -      | < noise and filtering
    -      |
    -      | < hybrid echo (2-way models)
    -     CO
    -      |
    -      | < A-law distortion + bulk delay
    -      |
    -     CO
    -      | < hybrid echo (2-way models)
    -      |
    -      | < noise and filtering
    -      |
    -      | < hybrid echo (2-way models)
    -    terminal