public void FirstDerivative()
 {
     IInterpolation ip = new StepInterpolation(_t, _y);
     Assert.That(ip.Differentiate(-3.0), Is.EqualTo(0.0));
     Assert.That(ip.Differentiate(-2.0), Is.EqualTo(double.NaN));
     Assert.That(ip.Differentiate(-1.5), Is.EqualTo(0.0));
     Assert.That(ip.Differentiate(-1.0), Is.EqualTo(double.NaN));
     Assert.That(ip.Differentiate(-0.5), Is.EqualTo(0.0));
     Assert.That(ip.Differentiate(0.0), Is.EqualTo(double.NaN));
     Assert.That(ip.Differentiate(0.5), Is.EqualTo(0.0));
     Assert.That(ip.Differentiate(1.0), Is.EqualTo(double.NaN));
     Assert.That(ip.Differentiate(2.0), Is.EqualTo(double.NaN));
     Assert.That(ip.Differentiate(3.0), Is.EqualTo(double.NaN));
     Assert.That(ip.Differentiate(4.0), Is.EqualTo(0.0));
 }