Add centre of mass in Detector class

Summary

Add centre of mass as property of the detector.

What is the current workaround to achieve the same functionality?

[ins] In [1]: det = kp.hardware.Detector(det_id=29)
++ Detector: Retrieving DETX with detector ID 29 from the database...
++ Detector: Parsing the DETX header
++ Detector: Reading PMT information...
++ Detector: Done.

[ins] In [2]: np.mean(det.pmts.pos, axis=0)
Out[2]: array([ 28.3       ,  18.7       , 111.28564516])