fix enrollment 2020 data

[almartin82]

description of the change

• fixes changed filepaths for 2020 enrollment files.
• refactors some of the df cleaning code to reduce the number of warnings in test_enr.R

how to test

you can run this test from test_enr.R. goal is to get all of those tests passing, this gets us closer.

  enr_2020 <- fetch_enr(2020, tidy = TRUE)

  expect_equal(filter(enr_2020,
                      district_id == '3570',
                      school_id == '999',
                      grade_level == "TOTAL",
                      subgroup == "total_enrollment") %>%
                 pull(n_students),
               36676)

  expect_equal(filter(enr_2020,
                      district_id == '3570',
                      school_id == '303',
                      grade_level == "01",
                      subgroup == "total_enrollment") %>%
                 pull(n_students),
               91)

  expect_equal(filter(enr_2020,
                      district_id == '3570',
                      school_id == '004',
                      program_code == "55",
                      subgroup == "migrant") %>%
                 pull(n_students),
               0)

[almartin82]

@chrishaid review this third (merges into master)