static double rmse(final Double[] calculated, final Double[] expected) {
		return sqrt(mse(calculated, expected));
	}
Root-mean-square error is measured as the average of squared difference
between predictions and actual observations.

@see #mse(Double[], Double[])

@param calculated the function values calculated with the current program
       tree
@param expected the expected function value as given by the sample points
@return the mean square error
@throws IllegalArgumentException if the length of the two arrays is not
        equal
@throws NullPointerException if one of the {@code double[]} arrays is
        {@code null}
jenetics.prog/src/main/java/io/jenetics/prog/regression/LossFunction.java