Support specifying hierarchical concepts

[ronaldtse]

In Metanorma they are displayed like this. The first instance is a higher-level concept, the next 2 are lower-level concepts that belong to the former.

Conceptually, a higher-level concept fully contains a lower-level concept. i.e. the lower-level concept is a specialization of the higher-level one.

In Glossarist YAML we must have a way of encoding this.

[ronaldtse]

As per https://github.com/metanorma/iso-10303-2/commit/e835891c9a7d4a4e5ed1d1dcd54ddf536099d49a, I have removed the hierarchical concepts from ISO 10303-2 for now because we do not want to delay publication due to this current task.