Function friedman_test 

pub fn friedman_test(
    performance_matrix: &Array2<f64>,
    alpha: f64,
) -> Result<StatisticalTestResult>

Friedman test for comparing multiple models across multiple datasets

This is a non-parametric test for:

• Comparing k models on n datasets
• Testing whether models have significantly different performance ranks
• Extension of Wilcoxon test to more than 2 models