A function used internally in extract_from_prescription to
parse the dosage units, such as millilitres, tablets, grams and so on.
If there are multiple units mentioned in a string, only the first is returned.
extract_dose_unit(txt)a character vector
A character vector the same length as txt, containing
standardised units, or NA if no units were found in the prescription.
A simple wrapper around str_replace_all and
str_extract.
Based on add_dose_unit.py from original Python/Java algorithm.