EML

Under
eml:eml/dataset/dataTable/attributeList/attribute/measurementScale/datetime,
there is a sequence of 3 required sub-elements:

- formatString
- dateTimePrecision
- dateTimeDomain

...but all the content of dateTimeDomain is optional, so to get a valid
document, it is necessary to add an empty dateTimeDomain element.

Presumably, to avoid this, the dateTimeDomain element should be optional