When Average Cost updates on 0 Inventory Lines, take Tax Status into account

The average cost fields update automatically when an item has 0 inventory, and the priority supplier cost is updated. This sets both the Avg cost (ex VAT) and Avg cost (incl VAT) to match that supplier cost.

Since the supplier cost we enter is exclusive of VAT, we'd want the system to look at the Tax Status field for that product, decide whether to add on a % for VAT and then use that as the Avg cost (incl VAT).