Add additional detector motor in area_detector_calib()

[carnisj]

When the instrument has a common rotation circle for the sample and detector goniometer circles (rotation of the whole diffractometer around the same axis as the detector inner rotation axis), it is not possible to include this additional motor in area_detector_calib().

[dkriegner]

I was thinking about this issue several times. Its a completely valid point of course.
However, its somewhat complicated to fix. The fitting in area_detector_calib is already now not very efficient/stable and likely a new interface needs to be defined for these functions.

For longer time I did not actively use this part of the code. So its unlikely to be fixed any time soon.