Fixed FLUX_HYBRID and MAG_HYBRID centering issues.

Added 2nd order moments measurements of the convolved model.
Fixed issues introduced in a recent updates with check-images.
Implemented new algorithms for SPREAD_MODEL and SPREADERR_MODEL (no optimization yet and the scaling is still variable however).
Added experimental SNR_WIN measurement parameter.
Fixed crash with empty ASSOC lists.
Pushed version number to 2.13.6.