Merge PO with sale_dropshipping

Updates on sale_dropshipping to :

do not merge orders with different destination addresses
populate the destination address when merging two purchase orders
Original Launchpad MP : https://code.launchpad.net/~camptocamp/sale-wkfl/7.0-dropshipping-merge-po-lep/+merge/216756

PR rewrited from https://github.com/OCA/sale-workflow/pull/25