[partner_subaccount]: Fix: if partner account is already present return the id of this account.
Refactoring: substituted get_create_supplier_partner_account() and get_create_customer_partner_account() with generic get_create_partner_account()
[data_migration]: Added control (in strict mode) for duplicated partners
Merge branch 'master' of https://github.com/iw3hxn/LibrERP