Save the values in volt-format.

def save_volt(elecs, volt, filename):
    """Save the values in volt-format.
    """
    # bring data in shape
    content = np.column_stack((elecs, volt, np.zeros(len(volt))))

    # save datapoints
    with open(filename, 'w') as fid:
        fid.write('{0}\n'.format(content.shape[0]))
    with open(filename, 'ab') as fid:
        np.savetxt(fid, np.array(content), fmt='%i %i %f %f')