calculateTax
Determines tax rates for all line items of the basket. Uses the shipping addresses
associated with the basket shipments to determine the appropriate tax jurisdiction.
Uses the tax class assigned to products and shipping methods to lookup tax rates.
Sets the tax-related fields of the line items.
Handles gift certificates, which aren't taxable.
Note that the function implements a fallback to the default tax jurisdiction
if no other jurisdiction matches the specified shipping location/shipping address.
Note that the function implements a fallback to the default tax class if a
product or a shipping method does explicitly define a tax class.