Data files missing for urca chapter

[jtr13]

GA fails trying to open files: https://github.com/jtr13/cc21fall2/actions/runs/3278238957/jobs/5396410329#step:8:2845 and
https://github.com/jtr13/cc21fall2/actions/runs/3278238957/jobs/5396410329#step:8:2849 in this chapter:

https://jtr13.github.io/cc21fall2/urca-unit-root-test-and-cointegration-test.html

@yzh9810 If you can make the eth-usdt.csv files available either on another external site or in a resources folder, we can change eval to true for all code chunks.