Merge remote-tracking branch 'upstream/master'

Conflicts:
	doc/source/calibration.rst
adding documentation
Merge remote-tracking branch 'upstream/master'
Merge remote-tracking branch 'upstream/master'

Conflicts:
	doc/source/calibration.rst
Test on linking the keypoints
Adding a function to plot the neighborhood
Implementing gradient field
Merge remote-tracking branch 'lintaillefer/master'

Conflicts:
	pyFAI-src/blob_detection.py
working on gradient field
 implementing the calculation of field of gradient
Merge remote-tracking branch 'upstream/master'
Merge remote-tracking branch 'aurore/master'