Add orientation sensor (IMU) as an assembly condition for IK

There is already a Markers assembly condition used in IK for position sensors. This PR adds an analogous assembly condition for orientation sensors, typically IMUs.
Sorry, my bad (or maybe Paul's bad) on the lack of documentation. The internal method it uses is well documented [here](https://github.com/simbody/simbody/blob/master/SimTKcommon/Mechanics/src/Quaternion.cpp#L45) and guarantees that the angle is -180 to 180. (The `Rotation` method punts to the `Quaternion` method.)