Complete Shunt regulation implementation

[cossartque]

When a shunt is not associated to a SMACC/ACMC in the database but is still with VoltageRegulatorOn=true in the iidm, the DYNModelCentralizedShuntsSectionControl cpp model should be used in the dyd for the regulation.

This regulation should be connected to the shunt (either directly to the Network model or to a Modelica model ShuntBWithSections).

There is an example in nrt/data/DynaFlow/CentralizedShuntsRegulation in dynawo (with Modelica model).

@marcochiaramello fyi

[lecourtoisflo]

In CentralizedShuntsRegulation nrt, no shunt is defined in the IIDM.