Dealing with pairs

VisuPairLink — V_Sim can draw link between nodes. This part defines a pair object and interface to draw pairs.
cylinder — Gives methods to draw cylinders as OpenGl objects pairing two elements.
wire — Gives methods to draw lines as OpenGl objects pairing two elements.

Drawing pairs is done via external modules. For the moment, two modules exist to draw pairs : one draws flat lines between elements (colour and width are user-defined) and the other use cylinders to represents pairs (radius and colour also user-defined).