Tame XML from Google Sheets API

Will be based on:

https://github.com/jennybc/manipulate-xml-with-purrr-dplyr-tidyr#readme

Overview:

• Wrangle XML from the Google Sheets API
• XPath to isolate XML nodesets as elements of a list
  • each element contains data for one row of a Google Sheet
  • wrapped in great gobs of thoroughly namespaced XML
• Use map() on that list with more XPath to retain only nodes from gsx namespace
  • these nodes contain actual cell data
• Use map() at depth 2 to extract sub-node names and text
• unnest() to go from one row per Sheet row to one row per Sheet cell