tdb/elife_upload: Update assay_date parsing

[joverlee521]

Description of proposed changes

Updates to the assay date parsing in tdb/elife_upload:

• make parsing of assay date from fstem a little more robust
• use the fstem's assay date as backup to the date that is in the measurement records.

Motivated by the latest discussion on ingesting the VIDRL flat files.

Checklist

• [ ] Checks pass