Improve the star/galaxy separation for validate_drp.

Many of the LSST SRD KPMs are defined for bright, isolated stars. There is clear evidence that galaxies are being included in current runs (they have significantly higher photometric scatter at the same mag|SNR). Improved star/galaxy separation will help generate better numbers

Stretch goal: Include additional informative plots about how well the `extendedness` value in the catalogs is successfully separating stars and galaxies.