private void rellenarTxtTotales()
 {
     factura.CalcularSubtotal();
     factura.CalcularIva();
     factura.CalcularValorTotal();
     IvaTxt.Text      = factura.IVA.ToString();
     SubTotalTxt.Text = factura.Subtotal.ToString();
     TotalTxt.Text    = factura.ValorTotal.ToString();
 }