Unify BehaviorRobot into floating-base robot paradigm (e.g. Tiago), perhaps through common base class

[cgokmen]

The BehaviorRobot behaves differently from the Tiago robot: Tiago keeps the real base link at the origin, but the BehaviorRobot moves it. We should unify. We should also get rid of the BRPart system as much as possible.

[ChengshuLi]

The command / control limits of the 6 DoF torso / arm / head joints are also wrong - the 3 DoF orientation should have range [-2 pi, 2 pi] rather than [-1000, 1000]. Currently robot_control_example.py is broken for BehaviorRobot.