[FIX] point_of_sale: rounding error

- Set 'Product Unit of Measure' to 5
- Set the UoM rounding to 0.001
- Create an excluded tax of 6 %
- Create a product costing 7.0
- Open the POS, add the product with a quantity of 0.535

The tax is 0.22 instead of 0.23.

The root cause of the issue is the in JS:

```
round_pr(0.535, Math.pow(10,-5)) * 7 = 3.744999999999999
round_pr(0.535, 0.00001) * 7 = 3.745
```

The numerical error impacts the tax computation later on.

As a workaround, we round by the maximum between the UoM rounding and
the 'Product Unit of Measure', which is what will be done in the backend
anyway.

opw-2205092

closes odoo/odoo#47372

X-original-commit: 125cdf96e94a94146804091ad47e701240c2529e
Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>

addons/point_of_sale/static/src/js/models.js

@@ -1517,9 +1517,9 @@ exports.Orderline = Backbone.Model.extend({
             var unit = this.get_unit();
             if(unit){
                 if (unit.rounding) {
-                    this.quantity    = round_pr(quant, unit.rounding);
                     var decimals = this.pos.dp['Product Unit of Measure'];
-                    this.quantity = round_di(this.quantity, decimals)
+                    var rounding = Math.max(unit.rounding, Math.pow(10, -decimals));
+                    this.quantity    = round_pr(quant, rounding);
                     this.quantityStr = field_utils.format.float(this.quantity, {digits: [69, decimals]});
                 } else {
                     this.quantity    = round_pr(quant, 1);