update chadwick register access

[bdilday]

Updates the reading of the chadwick register from read people files from the chadwickbureau. The register has changed file format from one file containing all people, to several files, e.g. people-0.csv.

This PR updates code to

• load the zip archive of chadwickbureau/register
• search the zip archive for people*.csv
  • instead of assuming path people.csv
• read them all into a dataframe

closes #308

[schorrm]

LGTM

[tjburch]

@bdilday can you re-up with master again to get the test updates from #318?

[tjburch]

Thanks!