Config breaks on upstream updates.

[giulioungaretti]

If a user had a custom config and we update upstream by adding a new field.
Their config value will remove the new field added upstream, thus breaking the import of qcodes!

The other way around is impossible to prevent though. If we remove a field upstream there's no way to delete it locally ,or even to raise a warning , but that's cool because maybe local uncommitted doe depends on it :D At worst it's a bit of extra noise!

[giulioungaretti]

thanks @eendebakpt and @nataliejpg and @MerlinSmiles for spotting my horrible 🐛