Add helper functions for RobotDARTSimu

[costashatz]

Add a few helper functions for RobotDARTSimu:

• Selecting the collision detector with a string and not DART code
• Add/Remove collision filter (mask collisions) without any DART code
• Custom collision checks, and casting of rays This is too much for now; I will make a different issue for this one.
• ...

@jbmouret @dalinel @PedroDesRobots let me know if you need more functions for the main simulator object. Recording videos/data of simulation will go in a different issue/PR..

[costashatz]

Fixed by #56...