exclude 'output' cells with plots from checking TODOs

[AgnieszkaZaba]

https://github.com/open-atmos/PySDM/actions/runs/19978973182/job/57301525326?pr=1750#step:4:1289

git_tracked_file = 'examples/PySDM_examples/Yang_et_al_2018/fig_2.ipynb'
gh_issues = {1: 'closed', 2: 'closed', 3: 'closed', 4: 'closed', ...}

    def test_todos_annotated(git_tracked_file, gh_issues):
        """raises assertion errors if a (TODO|FIXME) is not annotated or if the annotation
        does not point to an open issue"""
        if is_binary(git_tracked_file):
            pytest.skip("binary file")
        for line in _grep(git_tracked_file, r".*(TODO|FIXME).*"):
            if "(TODO|FIXME)" in line:
                continue
            match = re.search(r"(TODO|FIXME) #(\d+)", line)
            if match is None:
>               raise AssertionError(f"(TODO|FIXME) not annotated with issue id ({line})")
E               AssertionError: (TODO|FIXME) not annotated with issue id (      "image/png":