ImuSensor should also return accumulated orientation relative to initial imu reference frame

Issue #465 closed
John Hsu
created an issue

Add functions to return imu orientation:

/// \brief get orientation of the IMU relative to the reference pose
public: math::Quaternion GetOrientation() const;

/// \brief Sets the current pose as the  IMU reference pose
public: void SetReferencePose();

we need this to do away with computing IMU data from a link in drcsim.