Fix wrong documentation for exceptions in StatUtils and add missing

[dvmarcilio]

Hello! I found that a couple of methods in StatUtils were wrongly documenting in javadoc to throw MathIllegalArgumentException for null arrays, instead of NullArgumentException.
I took the opportunity to add documentation and tests for NotPositiveException for mode(double[], int, int).

As I mentioned in #206, I'm happy to submit more pull requests with similar fixes/additions to the documentation if it's something welcomed by the project.

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On another note, I found a few methods on Variance (called by StatUtils) that will throw NullPointerException (null dereference), instead of NullArgumentException. I'm also happy to submit a pull request fixing those. The fix is very easy but will entail other documentation modifications, and likely some test changes/additions.

commons-math/commons-math-legacy/src/main/java/org/apache/commons/math4/legacy/stat/descriptive/moment/Variance.java

Lines 368 to 371 in 57dda85

	public double evaluate(final double[] values, final double[] weights)
	throws MathIllegalArgumentException {
	return evaluate(values, weights, 0, values.length);
	}

commons-math/commons-math-legacy/src/main/java/org/apache/commons/math4/legacy/stat/descriptive/moment/Variance.java

Lines 452 to 454 in 57dda85

	public double evaluate(final double[] values, final double mean) throws MathIllegalArgumentException {
	return evaluate(values, mean, 0, values.length);
	}