Add flag for primary author

[eliselavy]

In order to generate correctly XML, see below:
https://journalofdigitalhistory.org/prerendered/en/article/dg/33pRxE2dtUHP

wrong order

[bendix3]

@eliselavy There are two main aspects to authorship, one is the order (e.g. “first author”) and one is the contact status (i.e. corresponding author). The order is defined by the order in which the <contrib> tags are placed. The contact author is defined by a corresp="yes" attribute in the <contrib> tag, but although I couldn't find this information in our XML guidelines section on contributor tags (http://www.wiki.degruyter.de/production/files/dg_xml_guidelines.xhtml#contrib), I believe an e-mail address is mandatory for authors tagged with the corresp attribute.

[eliselavy]

Thing about authors write several articles