Remove inner class MathUtil from class GeneralStatistics

[Richard Gomes]

Class MathUtil has methods min and max which should ideally be merged to similar methods in class Std.

In class Std min and max should take advantage of types T which implement Comparable<T>. This would probably make the code more flexible and the merge should make more sense.

[Richard Gomes (frgomes)]

I've migrated methods min() and max() from Std to Date.
The implementation in MathUtil remained as it is.
Class Std was removed as all its methods migrated to Array and/or Date.

In future, we need to study how these 'horizontal' methods should behave. I mean: there's a possibility that we find useful to define an interface Std which defines the behaviour of such methods min(), max(), lowerBound(), upperBound() and possibly others, pretty much like std:: library does. Then certain classes would ideally implement this interface.

At the moment, this need is not clear and, at the same time, a separate class Std does not look nice because break the encapsulation and responsibility chain of an OO design.