[SofaHelper] Moves operator>> specialisation for int from set.h to set.cpp

[damienmarchal]

Because I have so much branch with this change duplicated that it is worth having it in a PR and forget about that.

Of course, a real refactoring of the reading code would be better but until done let's just have this kind of cleaning .

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• it builds with SUCCESS for all platforms on the CI.
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• it does not generate new unit test failures.
• it does not generate new scene test failures.
• it does not break API compatibility.
• it is more than 1 week old (or has fast-merge label).

[damienmarchal]

[ci]-build][with-scene-tests][with-regressions]