makes addition and subtraction work also for ivs of different index

[mmschlk]

Motivation and Context

This PR allows adding two InteractionValues objects together wich have a different index. This previously was not possible and returned in an error. Now this will only throw a UserWarning but is technically possible. This could be helpful for experiments.

---

Public API Changes

• No Public API changes
• Yes, Public API changes (Details below)

---

How Has This Been Tested?

Existing Test has been updated to account for the correct indices in the objects and the UserWarning.

---

Checklist

• The changes have been tested locally.
• Documentation has been updated (if the public API or usage changes).
• An entry has been added to CHANGELOG.md (if relevant for users).
• The code follows the project's style guidelines.
• I have considered the impact of these changes on the public API.

---

[Copilot]

Pull Request Overview

This PR modifies the behavior of addition and subtraction operations between InteractionValues objects with different indices. Previously, such operations would raise a ValueError; now they emit a UserWarning and proceed with the operation, using the index from the first object.

• Changes error handling from ValueError to UserWarning for different indices during arithmetic operations
• Updates test to validate the new warning behavior and result index preservation
• Documents the change in CHANGELOG.md

Reviewed Changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 1 comment.

File	Description
src/shapiq/interaction_values.py	Replaces ValueError with UserWarning when adding InteractionValues with different indices
tests/shapiq/tests_unit/test_interaction_values.py	Updates test to expect warning instead of error and validates result properties
CHANGELOG.md	Documents the new addition/subtraction behavior for different indices