Single-use anchors/declared external anchors

[EfraimFeinstein]

In order to facilitate easier document merges:

1. Require that each anchor only be referenced once
2. Require that anchors that are (intended to be) referenced outside of the same resource be declared as type=external
3. Require that external anchors that have external pointers not be allowed to be removed from a document. Other xml:ids may be removed.
4. Change the TEI schema to reflect those changes
5. Change the validation routines to reflect those changes
6. Write an upgrade script for existing files.

This will be a breaking change, so it will require a "semi-major" version change.