Refactor RL IK into own extension

[juelg]

We currently have an interface for inverse kinematics which is implemented both by Pinocchio and the Robotics Library (RL). Since the RL comes with many system level dependencies it would be nice to refactor it into its own extension.

Definition of done

• rename the ik operation to "inverse" as the other one is called "forward"
• remove the RL ik and RL dependencies from RCS and test if it still compiles
• create a new C++ extension where the RL ik code is copied in, create python bindings and check that it compiles
• create documentation for the extension, especially naming the system level dependencies that need to be installed for it

[juelg]

implemented in #219