[FEATURE] Give out approximate results for powerflow at IterationDiverge and continue_on_error=True

[nitbharambe]

Problem

Extreme loading/generating situations result into voltage being in the non convergence region for newton raphson method. Such cases are common in planning operations.

It might be desirable for users to have Newton Raphson results if possible and linear results for cases which do not converge. Current method for this would be to run consecutive calculations: First full batch dataset, then linear powerflow on new dataset constructed from failed scenarios.

OpenDSS faced similar issue and chose a different method around it (https://opendss.epri.com/LoadModelsThatNearlyAlwaysConver.html). This signals some demand by the community.

Proposal

Give out linear power flow results for IterationDiverge cases and continue_on_error=True. We already initialize newton raphson with linear results so its a simple matter of caching that result. These scenarios still get classified as errors. But approximate results are available without reconstructing scenarios.

For automatic tap changing we still need to give out garbage value. (Or we can document that they are not useful)