[ADD] backport sepa direct debit

This is a backport of account_banking_pain_base, account_banking_sepa_direct_debit and account_banking_payment_export. Creating mandates and exporting payment order to PAIN works, but up to now I couldn't look into possible side effects of installing account_banking_payment_export
I don't understand your comment. Isn't this just a Python best practice, not Odoo specific?