Explain changes for final release of HED8.0.0 schema

[VisLab]

(Copied from #323 (comment) to begin new thread)

This probably would be a good place to explain why the top-level schema changed between versions 8.0.0-beta.5 and 8.0.0.

Before the release, we did an intensive review with three aims in mind:

1. Making sure that the current structure of the HED schema did not preclude overlaying a HED ontology for future linked data work.
2. Making sure that every term stood on its own --- that is if you saw the word without its hierarchy you would not misinterpret what it meant.
3. There weren't any terms in the schema that were obvious mistakes or in the wrong place.

Results:

• The review took longer than anticipated and ended up delaying the release of HED-3G to the beginning of August.

• The need to be compatible with a future ontology (an BFO-based OBO Foundry ontology, to be specific) triggered a top level reorganization. The main difficulty was that OBO has some requirements about relations, which were not satisfied in 8.0.0-beta. We ended up changing the top-level (Agent-property, Data-Property, Task-property, and Attribute) to (Property, and Relation).
  Note: As long as annotators use the recommended short-form, moving terms around doesn't affect the annotation, so this isn't as draconian as it might seem.

• There were some problematic individual terms that changed. For example, Left-side has become Left-side-of to make it clearly a Relation.
  Another problematic tag was Visual which became Visual-presentation. In general Sensory has been separated into subcategories Sensory-attribute and Sensory-presentation under Sensory-property. This was probably the change that will affect previously annotated data the most, but it cleaned up a lot of clarity issues.