debugging FrsSci code while running macros on pulser run

[audreychatillon]

Int_t -> UShort_t index to be consisten in a for loop

Describe your proposal.

Mention any issue this PR is resolved or is related to.

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Checklist:

• [ ] Rebased against dev branch
• [ ] My name is in the resp. CONTRIBUTORS/AUTHORS file
• [ ] Followed the pull request guidlines and the Git workflow
• [ ] Followed the seven rules of great commit messages