Fix broken XLink references

These local references are broken and their UUIDs don't exist anywhere
in the Donlon dataset.  We are replacing these by abstract references
for now, to restore the referential integrity of the XML document at
least formally.
Merge pull request #2 from m-click/fix-broken-xlinks

Fix broken XLink references

We will discuss later what is the best approach - only abstract references? only internal references? For the moment, it's good to fix the broken ones.