Node: add {get,find}ChildNamed() methods

Both methods look for a child with the given name as an immediate child of
the current node.

findChildNamed:

Will return an Optional containing the found node or empty.

getChildNames:

Is more insistent and will return the found node itself. If a node by that
name is not found, then a NodeException will be thrown.

src/main/java/net/kemitix/node/Node.java

@@ -127,6 +127,25 @@ public interface Node<T> {
     Optional<Node<T>> walkTree(final List<T> path);
 
     /**
+    /**
+     * Searches for a child with the name given.
+     *
+     * @param name the name of the child
+     *
+     * @return an Optional containing the child found or empty
+     */
+    Optional<Node<T>> findChildNamed(String name);
+
+    /**
+     * Returns the child with the given name. If one can't be found a
+     * NodeException is thrown.
+     *
+     * @param name the name of the child
+     *
+     * @return the node
+     */
+    Node<T> getChildNamed(String name);
+
      * Returns true if the Node has a name.
      *
      * @return true if the node has a name

src/main/java/net/kemitix/node/NodeItem.java

@@ -286,6 +286,25 @@ public class NodeItem<T> implements Node<T> {
     public Node<T> createChild(final T child) {
         if (child == null) {
             throw new NullPointerException("child");
+    @Override
+    public Optional<Node<T>> findChildNamed(final String named) {
+        if (named == null) {
+            throw new NullPointerException("name");
+        }
+        return children.stream()
+                       .filter((Node<T> t) -> t.getName().equals(named))
+                       .findAny();
+    }
+
+    @Override
+    public Node<T> getChildNamed(final String named) {
+        final Optional<Node<T>> optional = findChildNamed(named);
+        if (optional.isPresent()) {
+            return optional.get();
+        }
+        throw new NodeException("Named child not found");
+    }
+
     @Override
     public boolean isNamed() {
         return name != null && name.length() > 0;