ONIX 3 outputs: implement EDItEUR recommendations to improve 3.0 and 3.1 outputs

[tosteiner]

Describe the bug
Director of EDiTEUR, Graham Bell, has recently conducted a thorough review of our ONIX 3.0 implementation (the full Thoth spec). He kindly shared a number of suggestions to improve data quality even more. If these could be implemented in both 3.0 and 3.1 outputs going forward, that'd be fantastic!

Graham's review has been stored on our Nextcloud, under Collaborations/EdiTEUR, or here: https://cloud.thoth.pub/f/150708

[tosteiner]

Also adding this recent guidance on how to address different metadata fields in ONIX to meet new EU requirements & Validation of ONIX submisssions ... pasted into a gDoc for reference https://docs.google.com/document/d/1eGF3ieV6nxy--_MZwQ7vWlYUW0GcUBbLE8TXaNWVL-w/

[rhigman]

Also adding this recent guidance on how to address different metadata fields in ONIX to meet new EU requirements & Validation of ONIX submisssions ... pasted into a gDoc for reference https://docs.google.com/document/d/1eGF3ieV6nxy--_MZwQ7vWlYUW0GcUBbLE8TXaNWVL-w/

This is more relevant to #701, so I've added the link there.