Compute second moment of detected object

Compute second moment of a detected object in an AG image, and write this to the operational database.

The second moment should be suitably weighted. Quoting PFS-PFI-PRU101000-01 "The weighting function will probably be a suitably chosen Gaussian (but not chosen per-object as we need to calibrate how we estimate the defocus given these moments). It is possible that we will need to crudely estimate the PSF in order to set the weighting function."