Fix dependencies (I need the field "payment_order_type" which is in account_banking_payment)
Only use ASCII characters in the XML file (that's how banks use XML !)
Use the payment order reference as the Message Identification of the XML file.
Use the payment line sequence as the "End to End Identification"
Remove the Instruction Identification ('InstrId') because it is not a required field (only the End to End identification is required)
Rename variables in the function create_sepa() to include the number of each field as
used in the EPC guidelines
Reduce the number of flake8 warnings
Make code more modular :
- self.pool.get('banking.export.sepa').create() has a _prepare fonction
- most of the content of the XML file can be inherited via _prepare_field()
