[MIG] purchase: Migrate to new API

addons/purchase/__openerp__.py

addons/purchase/data/mail_template_data.xml

addons/purchase/data/purchase_data.xml

addons/purchase/data/purchase_data.yml

addons/purchase/data/purchase_demo.xml

addons/purchase/data/purchase_stock_demo.yml

addons/purchase/models/account_invoice.py

addons/purchase/models/mail_mail.py

addons/purchase/models/purchase.py

addons/purchase/models/res_company.py

addons/purchase/models/res_config.py

addons/purchase/models/res_partner.py

addons/purchase/models/stock.py

addons/purchase/report/purchase_order_templates.xml

addons/purchase/report/purchase_quotation_templates.xml

addons/purchase/report/purchase_report.py

addons/purchase/report/purchase_report_views.xml

addons/purchase/report/purchase_reports.xml

addons/purchase/security/purchase_security.xml

addons/purchase/test/average_price.yml