Possible ways to improve the schema by adding metadata #7

This is somewhat related to having a process to improve the schema (#6, item 1).

Right now the schema definition is simple and pragmatic, but I think it would be broadly useful to be able to add optional structured and unstructured metadata to XML-based databases.

Some possible examples:

• Where TDB lists of references are essentially string reference citations, we could opt for more structured metadata, like a bibtex string, bibtexml, or CFF (as string or XML-flavored)
• Phase metadata: phase name aliases, structural/symmetry information
• Metadata about the database itself, such as author information/contacts, version history, associated publications/citation information for the database itself
• User-facing documentation (similar to TDB DATABASE-INFO or the text after the > in the TDB PHASE command keyword)