EML

The current eml-coverage requires a bounding box described by two points. Many
ecological data sets are collected at a site with a point location but no know
bounding box. How can we accomodate point coverage? Two possibilities: 1)
change the content model to make one of the points in the bounding box optional,
or 2) change the documentation to tell the user to fill in identical points in
both bounding box coordinates if it is a point.