Getting the entity's relative name (short name).
Makes it possible to know the name of the entity. This name represents the path in the tree structure of the simulation, without PHX parent's full name.
E.g.: joint1/axis0/motor: it is the motor device loaded on the axis0 of the joint1
this path relates to RobotPHX, which contains this device.
E.g.: the robot "/robot1" contains the device "joint1/axis0/motor"