Investigate new segmentation image approach

[stscijgbot-rstdms]

Issue RCAL-953 was created on JIRA by Eddie Schlafly:

Multiband detection and segments are currently generated from a PSF convolved image.  Using the significance image (detection / detection uncertainty) probably does a better job in terms of accounting for known sources of noise, though may be problematic for measuring properties from those segments (e.g., the moments used for kron radii and fluxes).  Look at using the significance images for determining sources, segments, and shapes.