Capturing DOIs #16
Description
When present in a reference, the DOI should be captured and stored in the resolved_reference table in a separate column. Similarly for arXiv ids and ASCL ids. The best way to implement this, I think, is to add one new column (ARRAY(String)) called external_identifier and store these captured identifiers with a prefix prepended (e.g. "doi:" for DOIs). This way, if we decide to capture more identifiers, we can simply add them to the list with the right prefix.