HVDC AC emulation

[annetill]

Please check if the PR fulfills these requirements (please use '[x]' to check the checkboxes, or submit the PR and then click the checkboxes)

• 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 ? If so, link to this issue using '#XXX' and skip the rest

No. And I am not sure to be able to well describe the new feature ;-)

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

It models HVDCs in AC emulation.

What is the current behavior? (You can also link to an open issue here)

What is the new behavior (if this is a feature change)?

Does this PR introduce a breaking change or deprecate an API? If yes, check the following:

• The Breaking Change or Deprecated label has been added
• The migration guide has been updated in the github wiki (What changes might users need to make in their application due to this PR?)

Other information:

(if any of the questions/checkboxes don't apply, please delete them entirely)

[annetill]

I am going to write an issue because:

• this feature is only available in case of AC load flow, security analysis and sensitivity analysis ; but it will be easy to support DC calculations ;
• we don't support HVDC line contingency in AC security and sensitivity analyses ;
• an HVDC line in AC emulation is not seen as a branch, so it is not possible to create a factor with as reference branch an HVDC line.

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
2 Code Smells

90.5% Coverage
0.0% Duplication