[7.0] Allow EDI fields to be propagated if they have `_fnct_inv`

Odoo Issue: https://github.com/odoo/odoo/pull/4306

This is a rebase of 78520a2 from #114

Allow EDI fields to be propagated if they have `_fnct_inv`

Presently, edi will skip writing to function fields even if they have a `_fnct_inv`.

This is a problem with the [partner_firstname](https://github.com/OCA/partner-contact/tree/7.0/partner_firstname) which overwrites name with a field function feeding a required field.