Currently, the objectSizeStarSelector Task allows for source selection based on flux limits. A selection on signal-to-noise (S/N), i.e. flux/fluxErr, is often preferred, so should be added as an option for this source selector.

Additionally, there is currently no unittest for objectSizeStarSelector, so a minimal one should be added (following the test_astrometrySourceSelector.py and test_matcherSourceSelector.py examples).

For this ticket, the option will simply be added in the same manner as the flux selection is done, but all current defaults will be left as is. Looking towards the future, I anticipate submitting an RFC to make S/N selection the default (instead of a flux-based selection, but, in principle, both could be activated simultaneously) as well as adapting all the SourceSelectorTask to make use of the configuration-based selection classes in BaseSourceSelector-derived Tasks (e.g. objectSizeStarSelectorTask, AstrometrySourceSelectorTask, MatcherSourceSelectorTask, FlaggedSourceSelectorConfig) https://github.com/lsst/meas_algorithms/blob/master/python/lsst/meas/algorithms/sourceSelector.py#L153).