Grab citation info from retriever as part of downloading datasets

[ha0ye]

Current workflow:

1. use MATSS to call retriever to download data
2. use MATSS to generate an analysis project
3. run analysis project to get results and report.md

Updated workflow:

1. use MATSS to call retriever to download data and citation info.
2. use MATSS to generate an analysis project
3. run analysis project to get results and report.md and file with combined citations
4. report.md includes the combined citations file

[ha0ye]

status:

• citation from retriever is grabbed and stored in dataset/CITATION.
• when processed, we should add the citation as a text field under metadata.
  • processing code for BioTime is already doing this, but using the field name source, so we're renaming it to citation
  • some updates are needed to also include the citation for BioTime.
• with the resulting data objects, we need a function that extracts the citations, and combines them, removing duplicates
  • this can then be dropped in to the drake plan as a processing step