Add Morris In-Order Traversal Implementation

Fixes #626

Brief description

Added a space-efficient Morris In-Order Traversal implementation to the binary tree module, providing O(1) space complexity without using recursion or a stack.

Changes

• Implemented morris_in_order method in BinaryTreeTraversal class
• Added C++ backend support in BinaryTreeTraversal.hpp
• Added unit tests to verify correctness and tree structure preservation
• Fixed trailing whitespace issues in affected files

Implementation Details

• Uses threaded binary tree approach for constant space traversal
• Properly restores original tree structure after traversal
• Supports both Python and C++ backends
• Maintains compatibility with existing traversal interface

Testing

Added test cases that verify:

• Correct in-order traversal sequence
• Tree structure preservation after traversal
• Empty tree handling
• Node parameter handling
• Backend compatibility (Python/C++)