Add reciprocal for events, remove ProductBatch, units upgrade

EconomicEvent was the only place that hadn't had a separate reciprocal relationship added to it, in this case with Agreement. I think this addition makes the pattern match everywhere. Closes #718 (closed) .

Lot/batch was cleaned up and simplified. We were trying to stay synced up with DFC, but that has become difficult. dfc:ProductBatch was removed. A lot id will be stored in trackingIdentifier if it is needed. The lot expiration date was moved into EconomicResource, possibly causing occasional data duplication, but enabling some welcome simplification. The production date wasn't needed anyhow because we have that information in the production event information. Closes #702 (closed) .

Also we have a new example for tracking, closes #703 (closed) .

We added local overrides for Unit label and symbol, and clarified the documentation, closes #596 (closed) .