Best PA for guiding

I believe that this is now done, and tested via the OT and a test case.
(We may wish to add more test cases for this?)
We still have an issue with `ToContext.POS_ANGLE_CONSTRAINT_OP` though?  In `AgsServlet` `ToContext.apply` will be called and try to map `"UNKNOWN"` to a `PosAngleConstraint` which will blow up and result in a bad request response.  I think we can map `"UNKNOWN"` to `UNBOUNDED` there?