Returns and refunds don't work

[seand7565]

The returns and refunds system does not work. It's tied very heavily into the idea that the inventory_unit count is 1:1 with quantity, and major changes would be needed for it to work well with the inventory_unit change this gem makes.

Given that I want to merge this gem into master at some point in the future, it may make sense to overhaul returns & refunds on the Solidus side of things - see #4