Main changes
* DQ: Improved to_string() to be more similar to MATLAB.
* DQ_SerialMainpulator: removed support for dummy joints.
* Added DQ_SerialWholeBody. This class represents the serial connection of robots.
* Added DQ_SerialManipulatorDH. This class represents serial robots modeled using DH parameters and can consider prismatic joints.
* DQ_Robots: changed most serial manipulator robots to use DQ_SerialManipulatorDH instead of DQ_SerialManipulator. Changed KukaYoubotRobot to use DQ_SerialWholeBody instead of DQ_WholeBody.