Write groups under tag group instead of groupstree to enhance compatibility with previous versions

[tobiasdiez]

Right now, files written with 4.0 cannot be opened in older versions. The new format of the groups leads to a NPE.
As a way out, I changed the tag for the groups tree from groupstree to group (groups was already used by previous version way back). Thus older versions completely ignore the group tree and no longer try to parse it, so no NPE is thrown. On the downside, however, the groups tree is deleted upon save in the older version. Still a bit better than a crashing Jabref in my opinion.

• Change in CHANGELOG.md described
• Tests created for changes
• Screenshots added (for bigger UI changes)
• Manually tested changed features in running JabRef
• Check documentation status (Issue created for outdated help page at help.jabref.org?)
• If you changed the localization: Did you run gradle localizationUpdate?

[Siedlerchr]

On the downside, however, the groups tree is deleted upon save in the older version. Still a bit better than a crashing Jabref in my opinion.

I wonder why JabRef does not simply ignore the groups Tree then and deletes it on save.

[lenhard]

@tobiasdiez Even the old versions from 3.1 or 3.2 onward (not sure which) should not delete the new groups tree, since at some point we implemented the feature that JabRef does no longer delete random text from the bib file but keeps it. Anything prior to v3.0 will certainly delete it.

Anyway, I think this is a good compromise. Then people with older versions can still open the file and when they wonder that the groups tree is not correct (or gone), we can tell them to upgrade.

I just think you should use groups instead of group as the key in the meta data, since this is normally not about a tree consisting of a single group, but of multiple groups.

[tobiasdiez]

Well, even 3.8.2 deletes the group tag, probably since it is an unknown metadata item.

I could change group to groups but it was already used in a very (?) old JabRef version...not sure which.

[lenhard]

I see, with metadata the serialization might be different.

Hm, if it has been used before, we probably should not use groups. Since this name goes into the bib files of many people and will annoy us if we change it later, let us discuss a good version in the next devcall.

[koppor]

Decision: grouping

Alternatives: grouping, groupies, group, groups