[IMP][8.0] Extract method in l10n_nl_xaf_auditfile_export

[astirpe]

This PR is meant to simplify the way we can extend the auditfile template of module l10n_nl_xaf_auditfile_export.

Let's say I want to create a separate module to extend the auditfile template and, based on a certain condition, apply that extended template. For example, think about fixing issue #71 by creating a customized module.

In such cases, it would be very useful to have a simple method that can be easily extended, in order to retrieve the custom template.
This PR is a proposal to extract method def _get_auditfile_template() from the existing def button_generate().

[hbrunn]

from your code I understand that you need to pick a different template depending on values? Otherwise, I'd have suggested to make this a model method.

[astirpe]

@hbrunn Yes, in my case the template is based on a field of the record. I need to read some values from the singleton self. That's why you notice the @api.multi decorator.