CallDistanceTransceiver.py: use a min,max scheme instead of avg,uncertainty

CallDistanceTransceiver.py: use a min,max scheme instead of avg,uncertainty

Specify parameters using a minimum and a maximum values, like
call_setup_time_min,call_setup_time_max and ring_time_min,ring_time_max.

This makes it more intuitive to measure the stability of the line, and
makes the code more readable too.