unit_dispatch data stored twice in db

[AndreasEppler]

Each timestep of the unit_dispatch data is stored twice in the database:

In the UnitsOperator, the unit_dispatch that is then sent to the Output role consists of two entries each, one from the start timestep index and one from the end timestep index.

The next time, this function is called, the end index of the previous call becomes the start index of this call.

Therefore, each time step (except for the first and the last) for each unit is sent twice to the output role.
In the output role, we iterate over both time_values and append both to the records to then store them in the db.

This is, how the resulting dataframe filtered for one unit looks like:

[nick-harder]

@maurerle hey Florian, could you please take a look into this issue, if you have time?