Behaviour of initialization on previous values

[sylvlecl]

• Do you want to request a feature or report a bug?

Ambiguous.

• What is the current behavior?

When using PREVIOUS_VALUES as initialization option, we may get an exception with following message:

Voltage magnitude is undefined for bus '****'

This can make that option hard to use, because it can happen even after a very minor topology change.

• What is the expected behavior?

To be discussed: we might want to fill the holes when they occur ?
Or maybe we could fall back on a DC values initialization ?
Alternatively, maybe this issue must be left for the user to handle, through a try/catch for example, or ensuring that he uses that option only when he is sure to have results on all buses.

[geofjamg]

Similar issue than one described in #188

[annetill]

In case of any operator strategy in the security analysis, we have added this fall back (https://github.com/powsybl/powsybl-open-loadflow/blob/main/src/main/java/com/powsybl/openloadflow/network/util/PreviousValueVoltageInitializer.java). We can discuss now added this for the contingency state computation. @sylvlecl what do you think?