Duration for PEtabEvent

[sebapersson]

Currently PEtabEvent looks like:

struct PEtabEvent
    condtion
    affect
    target
end

For certain application it would be nice to also add a duration field stating for how long an event is applied. For example;

PEtabEvent(t == 3, S, duration=20)

would mean that a new value is added to S at t == 3, and then in this case at t == 2 the effect is reversed. Would only be applicable for events triggered at a specific time point, and for application where a state/parameter is set to a specific value (e.g. if the affect is to complicated deriving the reverse affect is hard).

At a lower level, this would entail building another callback with the reverse effect (invert the affect field), which should be quite straightforward.