Metaproteomics datasets annotation

Multiple users have been interested to annotated metaproteomics datasets. We have a first formalization of the use case here https://github.com/bigbio/proteomics-metadata-standard/blob/master/sdrf-proteomics/use-cases-under-development.adoc#metaproteomics-datasets but more information and examples are needed:

- [ ] A proper example is needed
- [ ] formalization about how to annotate multiple species is needed.

Start with sdrf-proteomics/use-cases-under-development.adoc and read the metaproteomics datasets use case. Gather the additional information and examples needed to document a proper example and formalize annotation for multiple species. Done means both checklist items are addressed in the use-case documentation.

Written by the indexing model from the issue text.