Do not store final frame if using save_frame_interval > 0 for Photonics2D

[gioelemo]

Description

This PR makes saving the final frame for the Photonics2D problem optional.

Fixes #212

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• I have run the pre-commit checks with pre-commit run --all-files
• I have run ruff check . and ruff format
• I have run mypy .
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

P.S. There are still mypy errors related to other problems (airfoil and thermoelastic2D) as described here which I think should be fixed.

@markfuge I can not assign you directly as reviewer as I am not an official developer of the project.

Reviewer Checklist:

• The content of this PR brings value to the community. It is not too specific to a particular use case.
• The tests and checks pass (linting, formatting, type checking). For a new problem, double check the github actions workflow to ensure the problem is being tested.
• The documentation is updated.
• The code is understandable and commented. No large code blocks are left unexplained, no huge file. Can I read and understand the code easily?
• There is no merge conflict.
• The changes are not breaking the existing results (datasets, training curves, etc.). If they do, is there a good reason for it? And is the associated problem version bumped?
• For a new problem, has the dataset been generated with our slurm script so we can re-generate it if needed? (This also ensures that the problem is running on the HPC.)
• For bugfixes, it is a robust fix and not a hacky workaround.

[markfuge]

Looks good to me @gioelemo
Some of the mypy checks don't pass but that is because of other problems and not your PR. We are fixing this also for another PR so we can merge this now and address the mypy things in a different PR