[FIX] Fix issue 973 : in case incoming picking are split, cost calculation is wrong

[cvinh]

in purchase_landed_cost, read_group function has to be "grouped by cost"

[pedrobaeza]

You need to rebase the PR to get only the relevant commit.

For the patch itself, we should rework it a little bit more:

• Quant domain should be obtained from stock.quant method quants_get_prefered_domain, called once per product (remember that the cost price is by product template, not by product variant), and combine results.
• I think fields appears added (with sum) when you don't group by that field, so please check that the result is not correct. If not, your patch is incorrect, because you need to sum all costs * qty.