Time Series as slack voltage preset

[t-ober]

In some cases we do not want the slack voltage to be fixed at a constant voltag e.g. 1 pu.

E.g.: When simulating training data, we want the slack node voltage to represent realistic values one would expect at that position.

Implement the option to pass in a time series that is used as the slack node voltage.

[staudtMarius]

@t-ober Maybe use could use this PSDM PR to read in a voltage time series.