extract information from metadata and add to output, to provide more context for data

[atn38]

After downloading the corresponding metadata for each dataset #15: we'll need to:

• extract attribute definitions and units from metadata and associate them with the correct entity/attributes
• extract publication dates and timestamps to help with versioning and checking for new updates (relates to #6)
• perhaps extract code definitions to help with flagged data (can be a separate sheet, can be comments, idk) #9