Constify arg and use const_iterator (BUGZ-4206)

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This merge request is automatically cherrypicked from !1782 (merged) from source branch https://source.coderefinery.org/lars_t_hansen/arc/-/tree/larstha-4206-issue12-const orginally made by @lars_t_hansen .
The original MR went to branch: https://source.coderefinery.org/nordugrid/arc/-/tree/master
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bz 4206 part 12: Argument of a function must be const for call from sibling function to work; as a consequence, body of function must also deal with the constness.