Add support for Petrinets

This release adds support for verifying petri net executions. Instead of providing a description for a Finite State Machine. We give it a petri net with an initial state and and execution tree. And the oracle will tell you if there exist a valid morphism between your inital state and your final state.

• Add --petri or -p options to the command line for petrinet executions,
• You can still use --fsm for finite state machines.