[IMP] product: Dyanamic d.p. on Volume field

Decimal precision for volume on products is always set to 2.
Some users that work on more precise volume would like to increase it.
In order to do it without customisation we add another system parameter
for Volume.

addons/product/data/product_data.xml

@@ -28,6 +28,10 @@
             <field name="name">Stock Weight</field>
             <field name="digits">2</field>
         </record>
+        <record forcecreate="True" id="decimal_volume" model="decimal.precision">
+            <field name="name">Volume</field>
+            <field name="digits">2</field>
+        </record>
         <record forcecreate="True" id="decimal_product_uom" model="decimal.precision">
             <field name="name">Product Unit of Measure</field>
             <field name="digits" eval="3"/>

addons/product/models/product_template.py

@@ -83,7 +83,7 @@ class ProductTemplate(models.Model):
         help = "Cost used for stock valuation in standard price and as a first price to set in average/FIFO.")
 
     volume = fields.Float(
-        'Volume', compute='_compute_volume', inverse='_set_volume', store=True)
+        'Volume', compute='_compute_volume', inverse='_set_volume', digits=dp.get_precision('Volume'), store=True)
     volume_uom_name = fields.Char(string='Volume unit of measure label', compute='_compute_volume_uom_name', default=_get_default_volume_uom)
     weight = fields.Float(
         'Weight', compute='_compute_weight', digits=dp.get_precision('Stock Weight'),