Rename age_low and age_high arguments

[gadenbuie]

What this

fcds %>%
  filter_age_groups(age_low = 20, age_high = 50)

is really saying is

fcds %>%
  filter_age_groups(greater_than = 20, less_than = 50)

or

fcds %>%
  filter_age_groups(gt = 20, lt = 50)

or

fcds %>%
  filter_age_groups(age_gt = 20, age_lt = 50)

[gadenbuie]

going with door number 4 because age_gt and age_lt are used in complete_age_groups() as well.