Make clean.sh scripts executable from any directory

[Eshaan-byte]

Summary

Extends the fix from #716 to the remaining case-level clean.sh scripts and one non-symlinked clean-tutorial.sh.

• Added SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)" and cd "$SCRIPT_DIR" to all 111 case-level clean.sh files
• Applied the same fix to partitioned-heat-conduction-direct/clean-tutorial.sh (real file, not a symlink, so Make clean-tutorial.sh executable from any directory #716 didn't cover it)
• Added missing set -e -u to flow-around-controlled-moving-cylinder/solid-python/clean.sh for consistency

Closes #667

Test plan

• Run ../clean-tutorial.sh from inside a case directory (e.g. quickstart/fluid-openfoam/)
• Run ./fluid-openfoam/clean.sh from a tutorial directory (e.g. quickstart/)
• Run clean-all.sh from the repository root
• Verify all three work without errors

[MakisH]

Hi @Eshaan-byte,

I think you meant to open a PR, not an issue. If there is one as well, I will eventually encounter that as well (difficult to keep an overview right now).