Converts raw NTEE codes into a normalized 3-character form used by all downstream classification functions. Handles padding, blank codes, specialty nonprofits (digits 2-3 of 01-19), and 5-character legacy codes.
Examples
get_clean_ntee( c("B29", "B0129", "B8443", "E0521", "", "A115") )
#> [1] "B29" "B29" "B84" "E21" "Z99" "A50"