[MTD] Separate odoo-version-specific code into a models/version_specific.py

[Nick-OpusVL]

This will localise rebase/cherry-pick/merge conflicts caused by different versions to only those places where version-specific stuff has actually changed. Most of the code here is not specific to Odoo version, and so should be kept in sync.

from . import whatever_introduces_the_original_model
def MtdOriginalModel(models.Model):
    _inherit='mtd.original.model'
    # implement the methods that are version-specific here.
    # e.g. how to compute the VAT results, and how to build the action to view the VAT breakdown