Suppress remaining `gcc` compiler warnings

[nickbianco]

Brief summary of changes

This PR is mainly intended to address any remaining gcc compiler warnings not suppressed in the previous two PRs (#3657 and #3658). The changes include:

• Adding sprintf to snprintf conversions that I missed in #3658.
• Fixed unused variable warnings. For variables that had no apparant use at all, I removed them entirely. For others in test cases, I replaced commented out cout statement with active logging statements.
• Fixed a boolean operation in CoordinateCouplerConstraint that using & instead of &&.
• Converted a size_t iterator to int in SmoothSegmentedFunction (to avoid an implicit conversion in the loop).

Testing I've completed

Looking for feedback on...

CHANGELOG.md (choose one)

• no need to update because...not user facing.

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This change is 

[adamkewley]

Overall, looks fine and you can merge it etc. but, just in case you end up refactoring stuff like this later, a safer pattern for snprintf with stack-allocated buffers is:

std::array<char, 128> buf;
std::snprintf(buf.data(), buf.size(), "blah %i", i);

Because there's one unambiguous location for the size of the buffer

(not dissing this PR: the existence of these snprintfs are clearly legacy)

[nickbianco]

I've addressed @adamkewley's specific comment. Let's switch to the safer buffer pattern in a future PR.