[ADD] new module 'product_taxes_group'

Hi all, 
I propose a new module named 'product_taxes_group' that create a new model 'tax_group' that is an association of customer taxes and supplier taxes. (managed by account manager members)
With that module, users have to select a tax group on product_template module instead of a group of taxes. (supplier and customer taxes).
So it is now forbidden to select incorrect combination of taxes. (For exemple in France, it is not possible to buy with a VAT of 5.5% and to sell with a VAT of 20%)

tax group support multi company. (you can create different tax group for different companies)

thanks for your review.
flake8: OK;
test coverage: ~80%
Regards.