[ADD] account banking sepa direct debit create mandates

Introduction
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When switching to SEPA payments, the mandates to be created can be a
serious show stopper. Usually, they are available in one form or the other,
but not in OpenERP. This plugin adds a button on payment orders to create
mandates for the current payment order where necessary.

Configuration
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By default, this module creates one-off mandates. If you rather want to create
recurring mandates (i.e. because your available mandates are recurring ones),
set the config parameter
'account.banking.sepa.direct.debit.create.mandates' to 'recurrent'.

Attention
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Only use this if you actually have the mandates!
Better put `needed` instead of `necessary`, don't you think?