High precision PDF for the Normal distribution

The normal distribution PDF is computed using:

Math.exp(-0.5 * x * x) / Math.sqrt(2 * Math.PI)

The value x^2 can be computed to extended precision. This extra information in the round-off bits can increase the accuracy of the exponential function (see NUMBERS-177 under the title 'Accurate scaling by exp(z*z)').

The effect of including the round-off bits on both accuracy and speed should be investigated.