[Topology] Add a "bus branch" topology kind

[sylvlecl]

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

Feature.

• What is the current behavior?

Voltage levels exhibit 3 topology views, but actual instances can only be of 2 kinds. Views types are defined by topology levels: NODE_BREAKER, BUS_BREAKER, BUS_BRANCH While kinds are: NODE_BREAKER, BUS_BREAKER

Therefore, we cannot have pure "bus branch" voltage levels, but only a bus branch view of a node breaker or bus breaker level. For those topology kinds, possible topologies of the voltage level are constrained by the underlying topology description (in iidm-impl, node/switch graph or configured bus/switch graph).

• What is the expected behavior?

In some contexts, in particular research, it's convenient to be able to change the topology for a variant without any constraints. See for example that issue https://github.com/powsybl/pypowsybl/issues/80.

For example, we may want to define the topology of a voltage level by explicitly defining buses:

VoltageLevel vl = ...; // A voltage level with BUS_BRANCH topology kind
vl.getBusView().newBus()
                        .id(busId)
                        .addConnectable(generator)
                        .addBranch(line, Side.ONE)
                        .add();
...
vl.getBusView().removeBus(busId)

That would create a new Bus instance, only for the current variant.

For other topology kinds, those methods should throw an UnsupportedOperationException.

• What is the motivation / use case for changing the behavior?

Being able to define topology for a variant without being constrained by an underlying unmodifiable topology.