Unify comments fields

Hi,

It seems that comments fields were added to both Node and its specialisations Work and Copy, but they were labelled slightly differently. Node has `commentsExternal` and Work had `externalCommonts` [sic].  Copy had none.

Banjo only ever wrote to the Node comments, so I will remove the comments fields from Work.

@JonathanShaw can you please review and merge?

Huw