data handling for eml2 packages
new datapackage handler needs to be able to handle either 'in-line' data or
special 'urn/uri' description saved in 'online url' tag. Initially, we will just
use an 'id' like that in current use - e.g. "ecogrid://higgins.234.3"
This has the advange that same urn is used for local and metacat packages.