We don't allow variants to have zero weight or volume. But a DFC import
in production showed that some catalogs list products with zero weight.
Despite the products having a weight, it's simpler to treat these as
items.
The DFC doesn't actually specify which cup it means. I don't expect
anyone providing "cup" as unit to measure their produce and expect it to
calculate as a regular volume. It can just be seen as items.
We still have the scale stored in two places but in our current system
that's part of a unit's "id".
If the DFC adds that value to its standard then we can use it for lookup
and don't need to repeat it.
* https://github.com/datafoodconsortium/taxonomies/issues/7
OFN products and variants need more data like a price but the DFC
stores that in a different object. We may get a larger graph containing
that information but we don't have any test data yet.
