v2: Clarify initial value of event trigger functions after pre-equilibration

[dweindl]

In PEtab v2 events are to be handled during pre-equilibration. I think it should be stated explicitly what the initial value is after pre-equilibration.

My current interpretation of the specs is that the value from the end of any pre-equilibration is used (since only changes from the condition table are to be applied and time is reset after pre-equilibration).