Bep 1006 delivery days

Implements delivery days handling and stagger price rule in SDK. Also implements net / gross shipping costs.

Both, net/gross shipping costs, and default delivery work days are implemented on the aggregating Rules struct, with sensible default values.
This looks wrong, the shippable is never set on Product calculations. Is there a test that verifies the calculated costs are shippable for this scenario?

I checked the aggregation rule as well, it doesn't handle this, would be a violation of SRP/OC though.