Fix corner cases of zero impedance subgraph

[flo-dup]

Do you want to request a feature or report a bug?
Bug

What is the current behavior?
Some corner cases not covered:

1. VoltageControl::findVoltageControlsSortedByPriority only checks if the controlled bus is disabled or all the controller buses are disabled, and does not check LfBus::isGeneratorVoltageControlEnabled(). All the controller buses can be enabled but with all the connected generators in contingency.
2. For a newly disabled branch which is a voltage controller, the corresponding voltage control can be selected as the main one if its target voltage is the highest for its type. If it is the main one, we dismiss it later, in VoltageLevel::findVoltageControlsSortedByPriority: the control is considered disabled, and then we don't consider any voltage control of that type.

What is the motivation / use case for changing the behavior?
To support corner cases, and cover them in unit tests. Or clarify why it could not happen.