Fixed grate functions

[chrishaid]

Added 2020 and pointed older years to new URLs.

Note that 2020 spreadsheet lacks graduated and cohort counts

Also fixed all parsing errors caused by characters (N, *, -) in purportedly numeric columns like grad_count.

Finally, changed some code to use idiomatic tidyverse style (using .data[[i]] to access columns with quoted names). I left the loop, much to my chagrin.

[almartin82]

I put one test back in d8d8d03 - here's what it looked like on the console. looks ok!