#27 include element

[Mark Lentczner]

The include element shouldn't take a path, just a name.
The element is only allowed to include a subcontainer of the
current container, not some arbitrary file path (relative or
absolute)

[Mark Lentczner]

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the semantics of include need to be nailed down, especially for
arrays

once this is all done, the RelaxNG schema needs to be updated to
reflect includes