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.
When using R.convertRotationToAngleAxis(), how is the angle bounded?  Doxygen on the method had no info.  can the angle range from [-180, +180] ?  [0, 360]? I guess I'm wondering if this computes the "minimum feasible" angle.  -90 should have the same error as +90, but if -90 is expressed as +270 we'd be in trouble.