Status of msmod modification for leap second addition

Described in [the msmod_leap_second_addition  document](https://github.com/FDSN/OBS-standards/blob/main/other/msmod_leap_second_addition.md).  Wayne talked to Chad about this at AGU, there seems to be no technical block, just available time.  Wayne repinged on Chad in case he could add this before IASPEI (would simplify standards document)