Emit warning when outputting a running timer

[Erik Schnetter]

Outputting (or accessing) running timers can give wrong results. There should be a warning about this.

Keyword:

[Erik Schnetter]

Alternatively, make the timers give the correct result when outputting a running timer.

This may require implementing a function "StopStart" in a clock that stops and immediately restarts the clock, so that the clock's current value is correct.