BoxSampler to sampler uniformly from a box (or hyperrectangle)

Create a sampler to sample uniformly within a hyperrectangle.

Alternative names for a hyperrectangle are a box or an orthotope. Below I use the term box for simplicity and to match the BallSampler for an n-ball.
 

public abstract class BoxSampler implements 
        SharedStateSampler<BoxSampler> {
    public static BoxSampler of(double[] a,
                                double[] b,
                                UniformRandomProvider rng);
}

Inputs a and b are the minimum and maximum of the box in each dimension.

Sampling can be performed using the same method as the LineSampler but using a uniform deviate for each dimension (instead of the same deviate for all dimensions):

p = a + u * (b - a)
  = a + ub - ua
  = (1 - u)a + ub

This will produce the same results as using an instance of the the ContinuousUniformSampler for each dimension with all samplers using a common RNG.