improve LED finder accuracy

[mikeferguson]

The LED finder can be improved in a number of ways:

• [ ] we drop samples if the single most changed pixel has NAN coordinates -- should do a small search in the region, or even better, fit a circle to the highly-rated points and then compute a centroid off their collective positions (this would help with noise).
• [ ] currently, the LED finder only works on the RGB image. There are several ways in which the depth data may be used (or even better, in which the depth data and RGB data can be better aligned).