'Overwrite the read_group in order to sum the function field \'inventory_value\' in group by'

@api.model
def read_group(self, domain, fields, groupby, offset=0, limit=None, orderby=False, lazy=True):

    res = super(Quant, self).read_group(domain, fields, groupby, offset=offset, limit=limit, orderby=orderby, lazy=lazy)
    if ('inventory_value' in fields):
        for line in res:
            if ('__domain' in line):
                lines = self.search(line['__domain'])
                inv_value = 0.0
                for line2 in lines:
                    inv_value += line2.inventory_value
                line['inventory_value'] = inv_value
    return res
