[PWGCF] radial flow decorr task update to add C3 skewness as an observable and other binning changes

[somabhatta]

No description provided.

[github-actions]

O2 linter results: ❌ 0 errors, ⚠️ 0 warnings, 🔕 0 disabled

[alibuild]

Error while checking build/O2Physics/code-check for 2227b8a at 2026-09-10 04:22:

## sw/BUILD/O2Physics-code-check-latest/log
--
========== List of issues found ==========
++ echo 'Found 1 errors and 7 warnings.'
Found 1 errors and 7 warnings.
++ [[ 1 -gt 0 ]]
++ cat /sw/BUILD/3099949a1da4c06b455edd781827db24057aae2c/O2Physics-code-check/errors.txt
PWGCF/EbyEFluctuations/Tasks/radialFlowDecorr.cxx:58:1: error: included header tuple is not used directly [misc-include-cleaner,-warnings-as-errors]
++ [[ 7 -gt 0 ]]
++ cat /sw/BUILD/3099949a1da4c06b455edd781827db24057aae2c/O2Physics-code-check/warnings.txt
PWGCF/EbyEFluctuations/Tasks/radialFlowDecorr.cxx:82:56: warning: casting (double + 0.5) to integer leads to incorrect rounding; consider using lround (#include <cmath>) instead [bugprone-incorrect-roundings]
PWGCF/EbyEFluctuations/Tasks/radialFlowDecorr.cxx:2230:19: warning: statement should be inside braces [readability-braces-around-statements]
PWGCF/EbyEFluctuations/Tasks/radialFlowDecorr.cxx:2232:38: warning: statement should be inside braces [readability-braces-around-statements]
PWGCF/EbyEFluctuations/Tasks/radialFlowDecorr.cxx:2236:19: warning: statement should be inside braces [readability-braces-around-statements]
PWGCF/EbyEFluctuations/Tasks/radialFlowDecorr.cxx:2238:38: warning: statement should be inside braces [readability-braces-around-statements]
PWGCF/EbyEFluctuations/Tasks/radialFlowDecorr.cxx:2242:19: warning: statement should be inside braces [readability-braces-around-statements]
PWGCF/EbyEFluctuations/Tasks/radialFlowDecorr.cxx:2244:38: warning: statement should be inside braces [readability-braces-around-statements]
++ [[ 1 -gt 0 ]]
++ exit 1
--

Full log here.