This MR introduces a second way of calculating the base price of a resource using "Base Price Quantity Step".

Currently, for a resource, we could define on a Supply Line "Quantity Steps",  and set a price for each of these steps. Then, depending on the quantity of the resource, the price of the unique matching step would apply.

For exemple : 

| Quantity Step Range | Price for the step|
|------------|---------|
| 0 -> 10 | 10€ |
| 11 -> 20 | 9€ |
| 21 -> inf | 8€ |

With the current method, for an Order of 15 products, the 2 range "11 -> 20" would apply, the unit price for the products would be 9€, and the total price would be 15*9 = 135€.

The new method of calculating would apply the _Price for the step_ __to, and only to, all items of this step__. Which means, for an order of 15 products, the 10 first products would have a unit price of 10€, and the 5 next products would have a unit price of 9€, which makes in the end a total price of 145€, and a base price of 9.66€.

/reviewed-on nexedi/erp5!896