[python] UHI Refactoring: Slicing Logic, Equality Operator, and Negative Indexing

[siliataider]

This Pull request:

This PR introduces some refactoring to the current UHI implementation in ROOT:

• Updated the slicing logic to include or exclude flow bins as needed: setting with a scalar excludes flow bins; setting with an ndarray may or may not include flow bins depending on the shape of the array.
• Added an equality operator to validate histogram shape and values (this is particularily needed for testing).
• Implemented -1 to access the last valid bin in an axis.

Checklist:

• tested changes locally