@Override
    public double sd() {
        return PI_SQRT3 * scale;
    }
@Test
    public void testSd() {
        System.out.println("sd");
        LogisticDistribution instance = new LogisticDistribution(2.0, 1.0);
        instance.rand();
        assertEquals(Math.PI/Math.sqrt(3), instance.sd(), 1E-7);
    }