Generic univariate summary statistic objects.

### UnivariateStatistic API Usage Examples:

#### UnivariateStatistic:

`/* evaluation approach */
double[] values = new double[] { 1, 2, 3, 4, 5 };
UnivariateStatistic stat = new Mean();
System.out.println("mean = " + stat.evaluate(values));
`

#### StorelessUnivariateStatistic:

`/* incremental approach */
double[] values = new double[] { 1, 2, 3, 4, 5 };
StorelessUnivariateStatistic stat = new Mean();
System.out.println("mean before adding a value is NaN = " + stat.getResult());
for (int i = 0; i < values.length; i++) {
stat.increment(values[i]);
System.out.println("current mean = " + stat2.getResult());
}
stat.clear();
System.out.println("mean after clear is NaN = " + stat.getResult());`