cartwheel: eliminate subdirectories in cartwheel directory, try to clean
    up #include directives
kintree: add solve method to SpInertia
mechsim/notes: delete Quaternion article (moved to XRP wiki)
mechsim/notes: delete center of pressure article (moved to XRP wiki)
cartwheel: add torque limit to motor model (defaults to infinity)
cartwheel: RobotInfo: add function to calculate joint velocity