things to address

[joshyam-k]

• [x] dplyr is required for map2_dfr (probably for future_map_dfr too)
• [ ] need documentation for package data sets (also rename them to be more specific?)
• [ ] more robust unit testing
• [ ] fill out readme (design a hex?)
• [ ] change name of main function
• [x] keep package inside of harvard-ufds organization?
• [x] figure out how to incorporate plan() into package
• [x] make main function s3
• [x] allow new levels in population data