Fixed decimal handling in order quantities

[barbazul]

Also added test coverage to getSimpleQtyToShip and getQtyToInvoice

Description

I am using rounding to a large number of decimals (8) to orders that cannot be completed due to strict equal to zero comparisons on item qty

Fixed Issues (if relevant)

Fixes #14328

Manual testing scenarios

1. Complete an order with a product using qty = 4.4
2. Create an invoice for qty = 0.4
3. Refund the invoice
4. Create an invoice for qty = 4
5. Create a shipment for qty = 4
6. Order should be complete

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)