Separeted nodes, petrinets and settings modules. Changed 'type' variable for subclasses.

-> Separated modules: nodes, petrinets and settings.
-> Created Base classes for PetriNets, Places and Transitions.
-> Created subclasses for PNs, Places and Transitions.
-> Modified _can_connect method to be a method of each node that would function as source.
-> Right now, BasicPetriNets are created when creating any kind of PN.