fix a bug in rigid body align

there was a typo in _cdef_MeasureAngleAxisCluster.align().  x1 was
passed as both x1 and x2.

Also

1. Add tests for rigid body measure.align and transform.rotate. (this was how I found the bug)
2. Ensure exceptions are handled in some of the cdef functions.
3. Ensure findrotation returns a real object
4. Fix the documentation for rotations.rotate_aa.  The order of the inputs
is non-intuitive.