Add the ability to store cross-references on DataStore

[RobLBaker]

Find a place and way to store data store cross references in EML. Ultimately DataStore will/should be able to extract this information and automate making the links between cross-references if they are in metadata.

One idea is to put them in the citations section and perhaps give them a specific ID tag so that DataStore can identify which references are cross reference and which are not.