Porting of l10n_it_partially_deductible_vat

Basically the porting consists of making some methods compatible with 8.0 api.

Two of the changed methods are: `compute_all()` of model `account.tax` and `compute()` of model `account.invoice.tax`. Both methods are still written as 7.0 api but they are wrapped by 8.0 api methods with the same name; the wrapper methods are delegated to call their respective parent 8.0 api methods.

It would be great to rewrite the methods in pure 8.0 api style but I don't have time for that.

why?