Update `ExperimentsToEventsConverter` to changed initialization semantics

[dweindl]

After PEtab-dev/PEtab#645 ...

• ... the first periods of a PEtab experiment has to be applied before initial assignments are evaluated. That means, the changes have to be implemented as initial assignments instead of event assignments at the initial timepoint, or any pre-existing initial assignments have to be included in the event assignment.
• ... for any subsequent periods, the event assignments need to be updated. Compartment-size changes in PEtab no longer follow the SBML event assignment semantics. That means, we need event assignments for all concentration-based species inside such a compartment to preserve concentrations instead of amounts.