Storage: Allow overwriting reference datetime with itself when fixed

We fix the reference datetime as soon as get/set_timeseries_sec is called. We did not allow any further setting attemps on it, even when the value would end up not being changed.

When calling optimize() multiple times, e.g. for initial state estimation, we could end up setting the reference datetime after it has already been fixed by calling pre() multiple times. Working around this exception is very difficult as things are now. It is much easier to allow setting the reference datetime after it is fixed when the result means nothing will change.