Make example showing how "...on_change" works on pubs and subs

[trevorhardy]

HELICS supports not publishing or receiving a value interface update unless it deviates a certain amount from a previous value. For example, you can configure a publication to only publish a new value when it is 0.1 more than the last published value. Similarly, you can configure a subscription or input to only notify the federate of the received value if it is, say, 0.5 more than the last received value. We need an example showing how to set this up and use these configurations. The configuration names are "only_transmit_on_change" (for publication), "only_update_on_change" (for subscriptions) with both needing the "tolerance" value also defined.