Add network modification to hvdc line action

[pjeanmarie]

Please check if the PR fulfills these requirements

• [ ] The commit message follows our guidelines
• [x] 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?

What kind of change does this PR introduce?

feature

What is the current behavior?

HvdcLine.toModification raise not implemented exception.

What is the new behavior (if this is a feature change)? Add a NetworkModification for HvdcLine and its extension HvdcAngleDroopActivePowerControl and implement toModification for a HvdcAction using this new NetworkModification.

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

• [ ] Yes
• [x] No

If yes, please check if the following requirements are fulfilled

• [ ] The Breaking Change or Deprecated label has been added
• [ ] The migration steps are described in the following section

What changes might users need to make in their application due to this PR? (migration steps)

Other information: