Add helper to generate `Vats` based on receipt entry data.

[WhyNotHugo]

From Receipt:

If the taxpayer has taxes or pays VAT, you need to attach :class:~.Tax and/or :class:~.Vat instances to the Receipt.

Create a helper to generate the Vat instances. It should have a flag to, optionally, also update the Receipt instance's total_amount field.