Merge branch 'feature/four-bar-and-stuff' into 'master'

VirtualRobot/EndEffector/EndEffector.cpp

VirtualRobot/IK/CompositeDiffIK/CompositeDiffIK.cpp

VirtualRobot/IK/DifferentialIK.cpp

VirtualRobot/IK/JointLimitAvoidanceJacobi.cpp

VirtualRobot/Nodes/FourBar/Joint.cpp

VirtualRobot/Nodes/FourBar/Joint.h

VirtualRobot/Nodes/RobotNode.cpp

VirtualRobot/Nodes/RobotNode.h

VirtualRobot/Nodes/RobotNodeFourBar.cpp

VirtualRobot/XML/RobotIO.cpp

VirtualRobot/XML/RobotIO.h

VirtualRobot/data/robots/ArmarIII/ArmarIII-Head.xml

VirtualRobot/examples/RobotViewer/DiffIKWidget.cpp

VirtualRobot/tests/VirtualRobotGazeIKTest.cpp

python/four_bar_mechanism/equations.ipynb