Implement whole-body controllers for fixed and floating base robots

[diegoferigo]

For the users of Gym-Ignition, it would be helpful having generic implementations of whole-body torque controllers. I propose to develop three controllers:

1. Fixed base controller: canonical computed torque controller
2. Floating base controllers:
   1. Joint Space Feedback Linearization: same as 1 but for a floating base system
   2. Constrained Inverse Dynamics: assuming the existence on holonomic constrants, control both joints and floating base projecting the torques to the null space of the contacts

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• 1) Fixed base controller
• 2i) Joint Space Feedback Linearization
• 2ii) Constrained Inverse Dynamics

[diegoferigo]

A new controllers architecture has been implemented in #166 and #167. It is now possible to run controllers in a separate thread than Python, and we introduced handy C++ interfaced to implement and integrate within Ignition Gazebo new custom controllers.

So far we only have independent PID controllers #166 and a fixed-base computed torque controller #167. We will extend our controllers set progressively.