Fixes #29

[AhmedHamdiy]

In this Pull request,

• I have fix addChild: is not in sync with its implementation.
• I added parent attribute to the AVLNode and tested it.

[AhmedHamdiy]

@Ducasse Could you review this pr please

[jordanmontt]

Could you explain the changes in this PR?

[AhmedHamdiy]

The most significant changes include adding parent references to nodes, updating node addition methods to maintain parent relationships, and adding methods to retrieve all children of a node recursively.

Parent-Child Relationship Management:

• src/Containers-AVL-Tree/CTAVLAbstractNode.class.st: Added parent instance variable and methods to get and set the parent node. [1] [2]
• src/Containers-AVL-Tree/CTAVLNode.class.st: Updated left: and right: methods to set the parent of the child node when assigning left and right children. [1] [2]

Node Addition:

• src/Containers-AVL-Tree/CTAVLNilNode.class.st: Modified addChild: method to replace itself with a new node and set the parent of the new node.

Node Access Methods:

• src/Containers-AVL-Tree/CTAVLNode.class.st: Added allChildren method to collect all child nodes recursively.
• src/Containers-AVL-Tree/CTAVLTree.class.st: Added root and allChildren methods to access the root node and retrieve all children in the tree.

Tests:

• src/Containers-AVL-Tree-Tests/CTAVLTreeTest.class.st: Added tests for verifying parent-child relationships and ensuring the root node has no parent.