Node: remove{Child,Parent}(): split trees branches apart

removeChild(node): removes the node from children of the current node,
making the child node into a new root node.

removeParent(): removes the current from from it's parent's children,
making itself into a new root node.

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

@@ -162,4 +162,15 @@ public interface Node<T> {
      */
     boolean isNamed();
 
+    /**
+     * Remove the node from the children.
+     *
+     * @param node the node to be removed
+     */
+    void removeChild(Node<T> node);
+
+    /**
+     * Removes the parent from the node. Makes the node into a new root node.
+     */
+    void removeParent();
 }

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

@@ -305,6 +305,26 @@ public class NodeItem<T> implements Node<T> {
         throw new NodeException("Named child not found");
     }
 
+    @Override
+    public void removeChild(final Node<T> node) {
+        if (children.remove(node)) {
+            node.removeParent();
+        }
+    }
+
+    @Override
+    public void removeParent() {
+        if (parent != null) {
+            parent.removeChild(this);
+            parent = null;
+            if (nameSupplier == null) {
+                // this is now a root node, so must provide a default name
+                // supplier
+                nameSupplier = n -> null;
+            }
+        }
+    }
+
     @Override
     public String drawTree(final int depth) {
         final StringBuilder sb = new StringBuilder();