Fixes in EAD bioghist import/export, refs #7124 (updates #7 and #8)

- Don't add empty bioghist elements in EAD export
- Don't create new 'Untitled' actors if bioghist value match
  the history value of the creator in the same position
- Improve comments showing different options on import
Ah okay, makes sense. The only other suggestion I have is to add @param $key to the top of the function comments, because it wasn't very clear when reviewing this function what that parameter was used for until I did a test!

Looks good to me otherwise