Add Network class method to convert networks to subsystems

[HaSchneider]

I would appreciate when there would be a function to convert a network to an subsystem. Or is there already a hidden function for that?
I think an implementation should be straight forward:

1. Create a subsystem class
2. Iterate over all sinks and sources and create corresponding amount of interfaces
3. copy all components to subsystem
4. Reconnect all components in Subsystem. Only all connections to sinks and sources needs to connected to the subsystem interfaces.