Add PMD suppression.

src/main/java/edu/hm/hafner/echarts/TreeNode.java

@@ -86,6 +86,7 @@ public List<TreeNode> getChildren() {
      * children are now the children of the current node. This is repeated as long as there are nodes with only one
      * child (package nodes at the top of the hierarchy).
      */
+    @SuppressWarnings("PMD.UnusedAssignment") // false positive https://github.com/pmd/pmd/issues/4960
     public void collapsePackage() {
         while (getChildren().size() == 1) {
             TreeNode singleChild = getChildrenMap().values().iterator().next();