Fixing a problem that occurred when combining SimulationHistory objects are combined using the __add__ function.
It was reworked by making it a dictionary with as keys the id of the first run associated with the timestamps and the value being this list of timestamps. When saving, in the csv, the timestamps are saved where the rows match the 1st row of the 1st run id associated with the timestamps. For space-saving, the timestamps are saved only once and the date is removed from all but the 1st row of the timestamps.