Parse an ISO time, potentially from a calibration valid start time,
and convert to form suitable for use in a curated calibration filename.
- Parameters:
- 
- valid_startstr
- Validity start time in ISOT format. 
 
- Returns:
- 
- date_strstr
- Form suitable for use in a curated calibration file name in a
data package: YYYYMMDDTHHMMSS.