Specific flag to enable/disable static var compensator regulation

[Luma]

Describe the current behavior

Currently, the SVC regulation is expressed using an enum RegulationMode that has three values: VOLTAGE, REACTIVE_POWER,OFF. The regulation is disabled by setting the regulation mode to OFF. Doing this, we lose information about the original regulation mode.

Describe the expected behavior

We should be able to disable the regulation without losing information about the regulation mode defined for the SVC. In a similar way to what is done for tap changers, we propose to add a specific flag, isRegulating.

Adding the proposed flag would also imply removing the regulation mode OFF.

Describe the motivation

Separate regulation mode and regulation status.
Uniform handling of regulation status in all the elements that can participate in regulation.

Extra Information

For tap changers, this flag is defined in the interface TapChanger. Ratio and phase tap changers extend this interface. Currently, ratio tap changers can only control voltage (reactive power output at a terminal is being considered in #2758). Phase tap changers have a regulation mode with three options: CURRENT_LIMITER, ACTIVE_POWER_CONTROL, FIXED_TAP.

The phase tap changer regulation mode FIXED_TAP could be removed if all users of the Network API considered the isRegulating flag. mode = FIXED_TAP should be completely equivalent to isRegulating = false.