adding dataset to the brapi spec

[MFlores2021]

No description provided.

[Gabriel-Besombes]

Would a dataset simply be a fixed snapshot of a search result or something more complex?
Could we look to be interoperable with the dataverse API? (https://guides.dataverse.org/en/latest/api/index.html). In their case a dataset is a group of files with accompanying metadata.

[SCYarnes]

Experimental dataset is an import concept in DeltaBreed. At its most basic a dataset is a placeholder to name/aggregate an observation unit list and observation variable list. We offer UI tabular views and download of experimental datasets by entity. Integration of a dataset concept may help support other change requests, like #611, #612, and perhaps #575.