Purchase order line hide tax in report

This module makes the printed Purchase Order document tidier by removing the Taxes column when it carries no useful information.

• On the Purchase Order report, the Taxes column is hidden automatically when all order lines share the same tax group. If lines use different tax groups, the column stays visible.
• You can override this behaviour per tax group: on the tax group configuration screen there is a new option, "Show tax column in purchase report". When enabled for a tax group, the Taxes column is always shown on purchase orders using that group, even if all lines share the same taxes.
• The option is available both in the tax group list view (as a toggle) and in the tax group form view.

The module extends the purchase order and tax group models and modifies the standard purchase order report template. It does not change any amounts, taxes or accounting entries; it only controls the visibility of the Taxes column on the vendor-facing document.