Orientation and user-defined detectors

[kif]

Should address together the case NexusDetector & spline file. Currently the NexuDetector handles the orientation, not the spline file. Both could easily be flipped. It is probably better that the detector definition is always stored in native orientation and that the flipping is handled by the poni-file at the detector loading.

Nota: the corner order has to be inverted in the case orientation is 2 or 4 (it must remain clockwise for full pixel splitting to work)

TODO:

• [ ] Do no more save/load orientation into the HDF5 file
• [ ] Consider orientation in the case of spline files
• [ ] Consider orientation in the case of NexusDetector

[kif]

In https://github.com/silx-kit/pyFAI/blob/main/src/pyFAI/detectors/_common.py#L741 consider an extra argument correct_orientation which would flip the pixel coordinate AND change the order of the corners accordingly