Adding Petri net diagrams (#1667)

Dear Dusan,

I was wondering if you plan to add Petri nets to the available types of diagrams. They can be used for a whole lot of things, including designing and analyzing concurrently running systems, manufacturing, finite state machines with shared resources, etc. Almost none of the modeler tools that I know of include Petri nets, so it would be an awesome addition to your modeler, especially considering that you already included finite state machine and grafcet diagrams as well.

The good news is, that Petri nets have been standardized and it doesn't have a complicated notation, so it should be easy to implement. You can find some preliminary information about it here:

http://www.informatik.uni-hamburg.de/TGI/PetriNets/index.php

Thanks for considering this, it would really be great to have this in your modeler.