Add a "comment" generic field in the DatasetMetadata model

[pierrepo]

This optional comment field could integrate information we do not actually collect (like the domains in MMDB, the CATH classification in ATLAS...)

[pierrepo]

Maybe we could remove the keyword field that is barely used and store possible keywords in this comment field.

[pierrepo]

Also, extra_text or extra_info would be a better field name than comment

[pierrepo]

To discuss with you @Essmaw