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 write function to check when data was last updated and updates them if there have been new changes compared to the local version #6)
• perhaps extract code definitions to help with flagged data (can be a separate sheet, can be comments, idk) To include or omit flagged data? #9