comments on CDM-OBS mapping tables

What exactly do we understand under:

* header.station_record_number
* header.primary_station_id_scheme  (add SOT ID)
* header.report_duration

Suggestions:
* [time_quality](https://glamod.github.io/cdm-obs-documentation/tables/code_tables/time_quality/time_quality.html) does not have a `unchecked` flag