add an on/off switch to Pump.mo
add filter for on/off signal
add on-off control to PumpExample.mo
make the filter of on-off signal conditional too to avoid overhead.
Merge pull request #173 from open-ideas/issue172_OnOffPump

@Mathadon I would rather not connect realExpression1 to a filter. It would cause multiple filtering. I connected instead the on_internal to a filter and made the filter also conditional (only if use_onOffSignal). I will close this issue as I updated the code according to your comments.