BUG/FEAT: Add affiliation author label

I found some inconsistent behaviour of `corref` in PDF and webpage rendering.

Current behaviour:
AFAIK the `corref` in the template comments indicates it's a way of showing the primary affiliation of authors with multiple affiliations on the web. It is compulsory otherwise the web page won't build correctly.
In the PDF built this tag is not really used for the same thing it seems -
`corref` seems to link to correspondence author's affiliation, but there's no way to indicate correspondence author.
The PDF template just take the last author with `corref` tag as the correspondence author.

Expected behaviour:
`corref` indicates the primary affiliation of the correspondence author.

Desired behaviour:
`corref` indicates the correspondence author.
Add `primaff` to indicate the primary affiliation of any authors with multiple affiliations.