* 1.0
- Start release.
* 1.1
- Split code from test_avltree.cpp to have a test_avltnode.cpp to test
  ADT AVLTNode separatly from AVLTree.
* 1.2
- Corrected several bugs due to do not update link to parent.
* 1.3
- Reimplement the rotation algorithm to simplify the balance process.
* 1.4
- Removed optional arguments (due to clang issues).
- updated to course 23-24.
* 1.5
- Using refs only for nodes.
- Simplify to use only one test program.
* 1.6
- Changed the default value of AVL Tree::is_empty() to true so that the 
  test program does not fail from the beginning.
* 1.7
- Implemented AVLTNode in separate files.
- Added post-c to assure link to parent.
- Improved README.md
- Added separate tests for compute_min_max_branch_length.
- Move to AVLTNode the logic to maintain the links to the parent node of a child.
- Added CTests.
- Adapted to course 24-25.
* 1.8
- Split tests for node in separate test program.
* 1.9
- Improved separate testing for ONLY_BSTREE mode.
- Enhance AVLTree class documentation and improve method signatures to use 
  references instead of copying where appropriate.
* 1.9.1
- Add missing include for <cassert> in avltree_iterator_imp.hpp.
* 1.9.2
- Separate a test program for AVLTreeIterator so incremental coding for
  BSTree, AVLTree and AVLTreeIterator is allowed.
- Updated to course 25-26.
* 1.10
- Standardizing iterator.
- Refactor fold/unfold avltree methods to override c++ operators <</>>.
- Apply zero-rule to avltree class.