@Override
    public String toString(ToStringContext<NAMETYPE> context) {
        return "cell_order(" + argument.toString(context) + ", " + order + ")";
    }
The <i>cell_order</i> tensor function produces a tensor with the rank of each
value in the original tensor according to the given ordering

@author arnej
vespajlib/src/main/java/com/yahoo/tensor/functions/CellOrder.java