Extract objects and determine the average FWHM, ellipticity, and
position angle across a field _CALCULATE_SEEING_STATS_
Currently uses SEXTRACTOR for source extraction.
The SEXTRACTOR configuration file is found in $ORAC_DATA_CAL/extractor_seeing_stats.sex.
This primitive runs only when the CALCULATE_SEEING_STATS internal header is true.
This primitive only operates on the current Group file.