Implementation of HVDC set point change action

[Hadrien-Godard]

• Do you want to request a feature or report a bug?
  Feature.

• What is the current behavior?
  The action is not implemented.

• What is the expected behavior?
  The HVDC set point change action is implemented as an implementation of the Action class (see issue Security analysis with remedial actions #2015).
  This action consists in changing the set point of an HVDC line.
  A starting idea of possible implementation is:

class HvdcPointChangeAction extends Action {
    String hvdcId;
    double setPointValue;
}

• What is the motivation / use case for changing the behavior?
  Allow security analysis to take this action type into account.

[annetill]

Solved by #2454