Allow removal of an IIDM network subneworks #1079
Description
Describe the current behavior
As in Powsybl Java, create a method to allow the removal of an IIDM network subneworks: network.flatten()
Describe the expected behavior
It should be possible to remove the subnetworks of an IIDM network.
Describe the motivation
It is not possible to remove the subnetworks of an IIDM network.
Extra Information
The Network::flatten method exists in Java and the expected behavior for pypowsybl is the same.