DCAT-US modified element missing when data do not contain update date

[hmaier-fws]

Modified element missing as test data did not include revisions or updates, only
original publication. This is a required field therefore logic should probably include
whatever date is there for the primary citation if there are no revisions or updates.

Originally posted by @dwalt in #264 (comment)

[hmaier-fws]

@dwalt I agree. The DCAT-US modified field is described as:

Most recent date on which the dataset was changed, updated or modified.

Since DCAT-US does not distinguish between an initial creation date and any subsequent modification dates, then a creation date would be a valid choice.

Although, not all dates might be valid choices. The dateType codelist contains items such as: "next update", "unavailable", and a few others.

We might want to consider applying a preferential hierarchy to a selected subset of date types. E.g.:

• lastUpdate, description: "date identifies when resource was last updated
• revision, description: "date identifies when the resource was examined or re-examined and improved or amended"
• creation, description: "date identifies when the resource was brought into existence"
• others??

I'm also guessing that many staff will use (inappropriately) lastRevision which should be used since that "date identifies when resource was last reviewed", which does not imply that an update was actually made. Not sure what we want to do with that. (my preference is for fairly strict adherence to definitions)