Support non deductible taxes

Some taxes (purchase) are not fully deductible so this means that such tax is composed by two children. One for the deductible percentage as usual and a second for the non deductible percentage. The account for non deductible part should be the (expense) account of the line. For now we can only set it on the tax object but it is not very flexible and we should support to use the line account as tax account.

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I'm wondering if it would not be better to have a single tax and having a nondeductible percentage on it.
Also this percentage could be variable, for example it can be defined on the taxed product or the party.
E.g. in Belgium VAT on car, the percentage can be computed from the usage ratio of the car.

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assigned to @ced

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Also for companies that can not deduct taxes, we should have a flag on the configuration to set the nondeductible percentage to 100% on line.

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I choose to not yet define deductible on product or party because I'm not sure it is really useful.

review341801002 updated at https://codereview.tryton.org/341801002/#ps361421014

I choose to not yet define deductible on product or party because I'm not sure it is really useful.

From what I've seen it depends on the kind of the product but not on the party. Parties are subjected to special taxes, which is already covered by tax rules.

For me it makes sense to add the deductible percentage to the accounting_category and use it as default value for the line like we do with taxes.

The main reason to have this is because there are some taxes that can be deducted only for a percentage of its usage. For example, in Spain if you are self employed and work from home you can only deduct a percentage for home related expenses (rent, phone, water, internet, etc.) but you can deduct full amount of other taxes like third party services.

But I'm not sure that it is right to fix the rate on the product or the category. For me it is most of the time a decision based on many factors.

But I'm not sure that it is right to fix the rate on the product or the category. For me it is most of the time a decision based on many factors.

For me there are some kind of products that when you have fixed the rate you can use for any invoice. That's why I proposed to have a default value on the category.

In the example I posted, once the rate is fixed all products related to the same category will use the same factor and most of the expenses are recurring ones.

Of course you can leave it empty and decide on each invoice.

I think it will not be a good design because if the rate must be fixed, so user could not change it. Also it will integrate badly with the purchase workflow. So for lack of better I prefer nothing.

review341801002 updated at https://codereview.tryton.org/341801002/#ps353401002

review341801002 updated at https://codereview.tryton.org/341801002/#ps357601004

review341801002 updated at https://codereview.tryton.org/341801002/#ps348101002

New changeset 59093c3932e2 by Cédric Krier in branch 'default':
Add deductible rate of taxes to supplier invoice
https://hg.tryton.org/modules/account_invoice/rev/59093c3932e2

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closed

New changeset bf8cd50e1840 by Cédric Krier in branch 'default':
Add deductible rate of taxes to supplier invoice
https://hg.tryton.org/modules/account_product/rev/bf8cd50e1840

New changeset c68f60eec696 by Cédric Krier in branch 'default':
Add deductible rate of taxes to supplier invoice
https://hg.tryton.org/modules/purchase/rev/c68f60eec696

New changeset 6c104944b20e by Cédric Krier in branch 'default':
Add deductible rate of taxes to supplier invoice
https://hg.tryton.org/tryton-env/rev/6c104944b20e