Check/correct datetime formatting in GeoImageConstraints

Date-time formatting is very strict in the FathomNet REST API. For example, `2007-08-02T00:00:00.000Z` works while `2007-08-02` is rejected. This fact, in conjunction with #16, makes for some confusing error messages.

It would help users to add an explicit check or reformatting of date-time strings before sending them to the API.

Originated from #15 