Follow-up of #2082

As written previously, such a class is likely to be used at several places:

• in pyFAI-calib2 where the code is available in pyFAI.gui.helper.RingExtractor.RingExtractorThread
• in pyFAI-calib where the code is available in pyFAI.gui.cli_calibration.AbstractCalibration.extract_cpt