This block detects when a PID tuning period should start and end.
Specifically, the tuning period is triggered to begin when either tOn
or tOff becomes greater than 0.
The tuning period is triggered to end when either tOn
or tOff changes after the tuning period starts, as illustrated below:
J. Berner (2017). "Automatic Controller Tuning using Relay-based Model Identification." Department of Automatic Control, Lund University.