TEI P5: Support nested TEI documents

[jhellingman]

For multivolume works, it sometimes is needed to nest the top-level <TEI> element. Support this in a reasonable way.

• Use main TEI header for over-all output metadata.
• Mostly ignore subsequent TEI headers, but generate a clear demarcation in the output.
• Make sure unique ID's are generated for various items that used to be unique in a single document are generated uniquely for this case.

[jhellingman]

Several templates are locked to a root path from the top-level TEI element. These need to be adjusted.

• Footnotes (DONE)
• CSS generation (for image widths, etc.)