make tests work and add uniqueness of objects (as they were referenced by names in some cases)
Add PetriNet#merge(net2), merges two PetriNets into one

Merges the second PetriNet into the first one, Places and transitions
are seen as the same, if they have the same name and description,
Arcs do if they have the same name, description, source and destination.

returns the resulting net (but changes the net directly!)