Returns a character vector of the recognised units.
lookup_units()
lookup_units()
#> [1] "ng/L" "ug/L" "mg/L" "g/L" "kg/L" "pH"
#> [7] "degC" "C" "CFU/dL" "MPN/dL" "CFU/100mL" "MPN/100mL"
#> [13] "CFU/g" "MPN/g" "CFU/mL" "MPN/mL" "Col.unit" "Rel"
#> [19] "NTU" "m" "uS/cm"