Change calculation process to calculate prices in groups

[mattbrailsford]

Currently the order calculation process calculates all prices without discounts, and then calculates discounts, however it might be beneficial if we instead calculated prices in groups:

1. Order Lines (without adjustments, adjustments, with adjustments)
2. Subtotal (without adjustments, adjustments, with adjustments)
3. Shipping (without adjustments, adjustments, with adjustments)
4. Payments (without adjustments, adjustments, with adjustments)
5. Total (without adjustments, adjustments, with adjustments)

This way, if something like a shipping calculator needs to change it's shipping price based on an adjusted price it can do so.

Linked to issue #616

[mattbrailsford]

NB This would be a breaking change as we'd need to introduce new pipeline groups and I don't think it will be possible to keep the original groups.