Fix clone implementation.

src/main/java/ee/ut/cs/gitlab/maikov/matrix/Matrix.java

@@ -30,7 +30,7 @@ public class Matrix {
 
         this.rows = rows;
         this.cols = columns;
-        this.elements = elements;
+        this.elements = Arrays.copyOf(elements, elements.length);
     }
 
     protected int getIndex(int row, int col) {