Replace deprecated istrstream with istringstream

[denproc]

Changes in this pull request

This pull request addresses the deprecation of std::istrstream described in #1637 by replacing the function with the recommended std::istringstream in KeyParser.cxx and mode.cxx. This update improves compatibility with newer C++ standards and ensures better maintainability.

Testing performed

Tested on MacOS (Apple Silicon) and CUDA-enabled Linux PCs.

Related issues

Fixes #1637

Checklist before requesting a review

• I have performed a self-review of my code
• The code builds and runs on my machine
• documentation/release_XXX.md has been updated with any functionality change (if applicable)

Contribution Notes

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• The content of this Pull Request (the Contribution) is intentionally submitted for inclusion in STIR (the Work) under the terms and conditions of the Apache-2.0 License.
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[KrisThielemans]

Perfect. thanks!