Byauthor should be included when citing a chapter within a book with an editor

[dcpurton]

At present bookauthor and mainauthor are supported, but they are suppressed in they are the same as author. They should print even if they are the same when there is also a bookeditor or maineditor, respectively. Probably should cater for editor being used in a sensible way too.

See https://sblhs2.com/2019/01/10/citing-a-chapter-from-a-single-authored-work-with-editors/