[FIX] stock_account: do not sum price unit

The view yields one line per stock move, with a price/unit on each line.
It doesn't make sense to sum price/units, the avg will be only meaningful when
grouping per product but it's correct in the default state.

Closes #11309

addons/stock_account/wizard/stock_valuation_history.py

@@ -108,7 +108,7 @@ class stock_history(osv.osv):
         'product_categ_id': fields.many2one('product.category', 'Product Category', required=True),
         'quantity': fields.float('Product Quantity'),
         'date': fields.datetime('Operation Date'),
-        'price_unit_on_quant': fields.float('Value'),
+        'price_unit_on_quant': fields.float('Value', group_operator='avg'),
         'inventory_value': fields.function(_get_inventory_value, string="Inventory Value", type='float', readonly=True),
         'source': fields.char('Source')
     }