DM-29673 Add centre of mass centroiding to support donut images

[mfisherlevine]

SdssCentroids fit donuts very poorly, and a simple CoM
centroid provides a pretty robust measurement when seeded
with the position using the existing algorithm.

This is not expected to deal with overlapping donuts as it
is used to simply to give a cutout to CWFS analysis.

[timj]

Some quick comments. Don't call this a review because I haven't really looked at the algorithmic code.

[czwa]

If missing the side is bad, should you check for that, or will you just get an exception and return None, indicating that there was a problem? I'm also not clear what would cause this block to raise. _getCenterOfMass doesn't directly raise, and I don't see why anything else would raise either. Am I just missing something obvious>

[mfisherlevine]

It's not that bad, it just means the result won't be very good, and checking for it is definitely not easy (all of which we discussed OOB, but just noting here too). Also, downstream code with see that, and it's not a huge problem anyway (plus that code is responsible for passing the boxsize anyway).

You're right about the try, and I've removed it completely now (having run some tests to make sure I'm definitely OK if the boxsize goes off the edge of the chip).