[WIP] Handle fictitious bus injections

[jeandemanged]

Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

Does this PR already have an issue describing the problem?

No

What kind of change does this PR introduce?

Feature

What is the current behavior?

We don't support fictitious bus injections introduced in powsybl/powsybl-core#2001

What is the new behavior (if this is a feature change)?
We support fictitious bus injections

Does this PR introduce a breaking change or deprecate an API?

• No

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[vidaldid-rte]

Is thie fictive injection feature working well ?
I do not understand this implementation in mergedBus. If there are several buses are get (sets P0 to each bus) and set (returns the sum of bus's P0) be consistent ? Same for computedBus.

public double getFictitiousP0() {
    return buses.stream().map(Bus::getFictitiousP0).reduce(0.0, Double::sum);
}

@Override
public Bus setFictitiousP0(double p0) {
    buses.forEach(b -> b.setFictitiousP0(0.0));
    buses.iterator().next().setFictitiousP0(p0);
    return this;
}

EDIT: OK I now see how get and set can be consistent if there is no topology change. However, setting a fictive injection on a bus - not on a bus breaker or a node, seems very fragile to me and a source of bugs, as opposed to the unambiguous use of a fictive load that is attached to something that is not volatile like a merged bus.