Fixed bug: impossible to create new document_attachment_type from menu 'Paramètres/Technique/Document joint/Type de la pièce jointe' because field is required

Use @tagged() decorator instead, in v15

Fix bug in document_attachment_type.xml: hidden 'Model' field in form_view while it is required in model: thus set invisible='0'

Completed fundamental xcg odoo_module structure and applied black, prettier and isort

The rest is per-project, up to implementers to enlarge them as needed.

Note that the "datas_fname" context allow you to give a specific
name to the document.

The previous method was not properly augmenting an attachment list, leading to
errors when using the "send by email" dialog box.

You can attach multiple documents as an attachment to the mail template.
Documents will not be regenerated should they have already been
generated beforehand and stored within a field of the object.

Note that now we can use this method to retrieve metadata from attachments.

Fix unit tests