reorganise class hierarchy and add two tests:

test_accept_quantity_divergence_on_invoice_with_started_packing_list (only for sale, currently failing)
test_accept_quantity_divergence_on_invoice_with_stopped_packing_list (for sale and purchase)



git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@24974 20353a03-c40f-0410-a6d1-a30d3c3de9de