Opacity state slice

[Rodot-]

:pencil: Adds a geitem operation to the opacity state class which allows slicing and indexing by shell

Type: :rocket: feature The opacity state should be able to be sliced in order to run the Montecarlo on sub-slices of the geometry. Let me know if there are other features or considerations to think about.

:vertical_traffic_light: Testing

How did you test these changes?

• [x] Testing pipeline
• [ ] Other method (describe)
• [ ] My changes can't be tested (explain why)

:ballot_box_with_check: Checklist

• [x] I requested two reviewers for this pull request
• [ ] I updated the documentation according to my changes
• [ ] I built the documentation by applying the build_docs label

Note: If you are not allowed to perform any of these actions, ping (@) a contributor.

[tardis-bot]

*beep* *bop* Hi human, I ran ruff on the latest commit (df19baeff43f4e5a559ca365412c3e4713f806a4). Here are the outputs produced. Results can also be downloaded as artifacts here. Summarised output:

```diff ```

Complete output(might be large):

```diff ```

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 85.71429% with 2 lines in your changes missing coverage. Please review.

Project coverage is 69.35%. Comparing base (5e7a23b) to head (df19bae). Report is 5 commits behind head on master.

Files	Patch %	Lines
tardis/opacities/opacity_state.py	50.00%	1 Missing :warning:
tardis/transport/montecarlo/numba_interface.py	50.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2758 +/- ## ========================================== - Coverage 69.83% 69.35% -0.48% ========================================== Files 191 196 +5 Lines 15022 15011 -11 ========================================== - Hits 10490 10411 -79 - Misses 4532 4600 +68 ```

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