Add correlation tests for various scenarios

Author: kvadrik

src/test/java/com/thealgorithms/maths/CorrelationTest.java

@@ -9,34 +9,39 @@ public class CorrelationTest {
 
     public static final double DELTA = 1e-9;
 
+    // Regular correlation test
     public void testCorrelationFirst() {
         double[] x = {1, 2, 3, 4};
         double[] y = {7, 1, 4, 9};
         int n = 4;
         assertEquals(0.3319700011, Correlation.correlation(x, y, n), DELTA);
     }
 
+    // Regular correlation test (zero correlation)
     public void testCorrelationSecond() {
         double[] x = {1, 2, 3, 4};
         double[] y = {5, 0, 9, 2};
         int n = 4;
         assertEquals(0, Correlation.correlation(x, y, n), DELTA);
     }
 
+    // Correlation with a constant variable is taken to be zero
     public void testCorrelationConstant() {
         double[] x = {1, 2, 3};
         double[] y = {4, 4, 4};
         int n = 3;
         assertEquals(0, Correlation.correlation(x, y, n), DELTA);
     }
 
+    // Linear dependence gives correlation 1
     public void testCorrelationLinearDependence() {
         double[] x = {1, 2, 3, 4};
         double[] y = {6, 8, 10, 12};
         int n = 4;
         assertEquals(1, Correlation.correlation(x, y, n), DELTA);
     }
 
+    // Inverse linear dependence gives correlation -1
     public void testCorrelationInverseLinearDependence() {
         double[] x = {1, 2, 3, 4, 5};
         double[] y = {18, 15, 12, 9, 6};