reproin: study-convert option to consider accessions since the last converted date

in _scans.tsv we have a record of all acquired/converted dates. Assuming that we would want to consider only new ones, we can add a mode to look at the dates starting from the

```
find sub* -iname '*_scans.tsv' | xargs grep -v filename | awk '{print $2;}' | sort -n | tail -n 1
```