[PWGHF] updated to study multiplicity dependence

[sashingo]

• Add the correlation between centrality and FT0 multiplicity.
• Add the correlation between centrality and PV-track multiplicity.
• Store the PV and FT0 multiplicity distributions as a function of the collision vertex position.
• Add the correlation between EMC cluster energy and the selected event-activity estimator.
• Rename the existing centrality–multiplicity correlation histogram to clarify that it uses FT0 multiplicity.

[github-actions]

O2 linter results: ❌ 0 errors, ⚠️ 0 warnings, 🔕 1 disabled

[vkucera]

@sashingo updated to study multiplicity dependence where?

[sashingo]

@vkucera ,
actually I updated and added QA histograms for multiplicity-dependent studies in the task, while the event and electron selection criteria remain unchanged.

[alibuild]

Error while checking build/O2Physics/code-check for 3a5cd24 at 2026-08-06 23:39:

## sw/BUILD/O2Physics-code-check-latest/log
--
========== List of issues found ==========
++ echo 'Found 2 errors and 2 warnings.'
Found 2 errors and 2 warnings.
++ [[ 2 -gt 0 ]]
++ cat /sw/BUILD/9d3bacb38b49416646c0753d8e093f6f57c78d54/O2Physics-code-check/errors.txt
PWGHF/HFL/Tasks/taskElectronWeakBoson.cxx:181:92: error: both sides of overloaded operator are equivalent [misc-redundant-expression,-warnings-as-errors]
PWGHF/HFL/Tasks/taskElectronWeakBoson.cxx:511:13: error: do not declare C-style arrays, use 'std::array' instead [modernize-avoid-c-arrays,-warnings-as-errors]
++ [[ 2 -gt 0 ]]
++ cat /sw/BUILD/9d3bacb38b49416646c0753d8e093f6f57c78d54/O2Physics-code-check/warnings.txt
PWGHF/HFL/Tasks/taskElectronWeakBoson.cxx:620:42: warning: statement should be inside braces [readability-braces-around-statements]
PWGHF/HFL/Tasks/taskElectronWeakBoson.cxx:622:40: warning: statement should be inside braces [readability-braces-around-statements]
++ [[ 2 -gt 0 ]]
++ exit 1
--

Full log here.