Support for external references

[dleehr]

The final plant datasets for the Tree of Sex WG list the references as a short string, with longer citations in an external list. I received a sample dataset for Baker's Law WG and it does the same.

If this is a common practice, it might be a good idea to support it in TraitDB

[hlapp]

Yes, this is a pretty common practice, especially for datasets with multiple entries coming from the same reference.