Simplify article publication date elements in Citation

The Citation.citedArtifact.publicationForm element contains multiple 'date of publication' elements:

publicationDateYear 0..1 string

publicationDateMonth 0..1 string

publicationDateDay 0..1 string

publicationDateSeason 0..1 string

publicationDateText 0..1 stirng

articleDate 0..1 dateTime

This pattern was developed from earlier modeling that had separate 'publicationDate' concepts for a periodicRelease publicationForm (e.g. a journal issue) and for the individual article (articleDate), and earlier modeling in which sources may store year, month and day as separate elements without standardized data representation.

We now propose to simplify this by removing the publicationDateYear, publicationDateMonth, and publicationDateDay elements – they can all be converted and represented in dateTime format or unstructured string format within publicationDateText.