Fix StaticVarCompensator and VscConverter used as reference generator

[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?

Bug fix

What is the current behavior?

Static Var Compensator and VSC HVDC Converters are used in reference bus selection based on generator reference priorities.
Generator Reference priorities should use only real generators.

In the case of a slackDistributionFailureBehavior configured to mode DISTRIBUTE_ON_REFERENCE_GENERATOR:

• if a Static Var Compensator was selected, then slack is not really assigned (biggest issue)
• if a VSC HVDC Converter was selected, then slack could be assigned but this doesn't really sound right (this point is debatable).

What is the new behavior (if this is a feature change)?
Static Var Compensator and VSC HVDC Converters are not used anymore in reference bus selection based on generator reference priorities.
The issue of undistributed slack in the presence of an Static Var Compensator is fixed.

[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