NodeAccessor getitem

[p1c2u]

This pull request refactors the NodeAccessor and related classes in pathable/accessors.py to use the new __getitem__ method for path traversal instead of directly calling _get_node. This change improves code readability and consistency by centralizing node access logic in a single method.

Refactoring for centralized node access:

• Added a __getitem__ method to NodeAccessor that retrieves a node given a sequence of path parts, encapsulating path traversal logic.
• Replaced all direct calls to _get_node(self.node, parts) with self[parts] throughout the file, ensuring all node lookups go through the new __getitem__ method. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]

Documentation and comment updates:

• Updated comments to refer to the new access pattern (get() or self[parts]) instead of the old _get_node() method, reflecting the refactor. [1] [2]

This refactor makes the codebase easier to maintain and less error-prone by reducing duplication and clarifying the intended node access pattern.