DC attribute is not mapped in LoadFlowParameters

[obrix]

DC attribute is not mapped in LoadFlowParameters. In the load flow API we have two run functions, run_ac and run_dc, and the dc parameter is harcoded in those functions and not registered as a load flow parameters attribute.

This was not an issue until other parameters hosting an instance of load flow parameters are used with serialization / deserialization from json. For example the RaoParameters host a load flow parameters set in LoadFlowAndSensitivityParameters and when loaded from json the value for the dc attribute is lost.

The solution would be to properly map this dc attribute and to force it to a specific value in run_ac / run_dc, so other users of the load flow parameters mapping have a fully mapped parameters object.