- Move the time_util.c functions to the lib directory, and have the
    lib/logic/timer.c use them

  - Allow parameters to be specified by the values stored in plc points,
    and not only by constants (eg. P, I, D const. of the PID function block,
    and the multipliers of the add function block).

* - Have the module use the scan_period provided by the priod library, instead
    of handling the period all by itself using the usleep() function...

  - PID loop: add a bias/offset (fourth parameter)
              allow PID to work in series instead of parallel (configurable!)