Additive units

[rwilson4]

I'd like to be able to interpret quantities like "5 gallons 2 quarts 3 cups" as referring to the quantity 5 gallons plus 2 quarts plus 3 cups. I'd also like to be able to convert "5.6875 gallons" to "gallons quarts cups" with integer values of all but the last. So for example, convert("5.6875 gallons", "gallons quarts cups") should return the quantities above, perhaps as an array, while convert("5.6875 gallons", "gallons quarts") would return, perhaps, [5, 2.75] or "5 gallons 2.75 quarts".