Added new constructors to statistics-related utility classes that accept a java.util.Random object

Hello,

I've added new constructors to some statistics-related classes under the `radlab.rain.util` package that accept a `java.util.Random` object.
The rationale for this change is to enable the reuse of an already created random number generator and thus to help experiment's reproducibility (i.e., to reproduce experiments with the same random number sequence starting from the same seed).

Please, consider to merge this commit.

Best,

-- Marco