establish Dataset graph pattern

[mbjones]

This is to determine how the schema:Dataset, dcat:Dataset, and related classes inter-relate, and how they will be tied to People and other critical entities.

[mbjones]

The current description of the Dataset pattern is here:

https://github.com/DataONEorg/slinky/blob/main/docs/mappings.md

I think we should modify that document to describe the new schema:Dataset pattern we would like to follow