Add amount and amountAvailable
Ref. https://github.com/valueflows/process/issues/7#issuecomment-221329544.
I left onhand amount to be just vf:amount, which for whatever reason was not there yet. Do people agree this is a good name for the quantity and unit combination from qudt?
I added vf:amountAvailable even though it probably wouldn't be stored in the database, because it might be requested as data, so seemed appropriate for the vocab.
PR so you all can discuss.