Look for ac files in unannotated directory.

No longer require .ac symlinks, which are problematic on Windows
Update STAC discourse relations inventory

No more known unknows
Introduce notion of span equality
Perhaps better treatment of empty annotations

Introduce type distinction between leafs (terminals None)
and empty annotations (terminals is empty list)
Fix Glozz XML output for schemas
stac: Enhannce is_edu, add is_relation_instance

Take into account the two different annotation stages