* unit_price_price_and_unit refers UnitPrice which refers to other attributes
that belong to variant and product.
Instead of collecting all the attributes that were using indirectly by the method and adding them to the serializers
I perefered to serialize the result of the method call.
1. reimplment the helper on the LintItem presenter
2. add the tax rate associated to the line items to the list of objects to serialize during the order serialization.
3. implement a presenter for the introducer tax rates.
There are three main components:
1. The invoice model
2. order serializers: serialize the order for the invoice
3. data presenters: the object that will be use to access the order's serialize data
