Apply narrative schema in child documents

[jwoLondon]

Currently narrative schemas can only be linked a 'root' document. If a document follows another, it cannot directly link to a schema in its own frontmatter.

A consequence of this is that any schema requirements apply to all documents in a linked or branching narrative, which limits the usefulness of the schema for guiding different branches of a narrative.

A common use case which we cannot currently support is to have a root document containing some literate data shaping and a following document containing a narrative about its visualization structured by a schema.

We need a mechanism for scoping schemas on a per-document basis. Ideally we would allow schemas to apply only in the document in which the schema is referenced in its frontmatter, and that we would allow schemas to be referenced in non-root documents as well as the root.

If this is not practical to implement, we could keep the current root-only schema referencing but allow a per-document ignoreSchema schemaName option to prevent schema warnings from appearing in documents in which we don't want them to be applied.