sub-exposure velocities in APOGEE

Adrian Price-Whelan (Flatiron) opened up the black box of the SDSS-IV APOGEE data, looking at whether we can measure stellar radial velocities in the short (9-min) sub-exposures that make up a full (1-ish hour) APOGEE exposure. We pair-coded for much of the morning and showed that yes, yes we can! This conceivably increases the time resolution of the APOGEE data considerably, and is useful for short-period systems (and, I hope, red-giant asteroseismology). We have to figure out what's our best method for making the measurements now.