Add Neighbor Joining widget

[drejcbevk]

Issue

Adds a prototype Neighbor Joining widget.

Description of changes

Added files:

• neighbor_joining.py
  Implements the core algorithm, TreeNode structure, leaf ordering helpers and Newick export.

• neighbor_joining_adapter.py
  Converts the TreeNode structure into Orange's hierarchical.Tree structure.

• dendrogram_nj.py
  Custom dendrogram component, supports non-ultrametric trees by preserving leaf heights and branch lengths.

• owneighborjoining.py
  Adds the Orange widget for visualizing, pruning, selecting, and outputting clusters.

• test_neighbor_joining.py
  Tests for core algorithm and tree adapter.

• test_owneighborjoining.py
  Widget tests for input handling, error states, tree construction, and outputs.

Includes

• Code changes
• Tests
• Documentation