Modified date of vocabulary lost

[dalito]

@nmoust - FYI

In nfdi4cat/voc4cat#12 the "modified date" present in the submitted Excel file was lost in the conversion to SKOS. The SKOS concept scheme uses the same value for "modified date" as for "created date". This may be a bug in the convert sub-cmd.

[dalito]

This behavior comes from the code merged from vocexcel. It uses the attribute created if a modified date is present (which seems like a bug). Interestingly no one noticed this until now. A fix is to simply use the modified attribute instead.

However, we should take the chance to re-consider how we want to keep date/version in Excel/rdf and git-tags automatically in sync instead of relying on a version/date entered in Excel.