Add 2019 grate

[chiouey]

proposed changes

• add new groups to grad_file_group_cleanup
  • 2019 combines asian and pacific islander
• format get_raw_grad_file logic
• add new urls for 2019 4y and 2018 5y
• add 5y grad rate column names to tidy_new_format()
• divide 5y grad rates in years 2012, 15-18
• add ground truth tests for 2019 4y and 2018 5y
• modify grad counts function and tests to acceptable years
  • 2011 file only has rate, prior to 2010 files have moved/haven't been found

how to test

testthat::test_file("tests/testthat/test_grate.R")