Helper function to grab the conversion factor from an input unit and given list
find_factor(full_unit, units = NULL, prefix = "^")
full unit, e.g. "mL/min/kg"
unit specification list, e.g. `list("ml" = 1/1000, "dl" = 1/10, "l" = 1)`
prefix used in matching units, e.g. "^" only matches at start of string while "_" matches units specified as "/"