Add DATE-OBS as optional keyword

[kelle]

I noticed that DATE-OBS is not in the table of metadata fields in Spectrum DM v1.2. There is TMID in MJD but no keyword for the date of the observation. I propose that DATE-OBS be added. Maybe as Char.TimeAxis.Coverage.Bounds.Start since it's supposed to be the date at the start of the observation.

(Follow up from convo started on Slack.)