[PWGLF,PWGMM] Fix flattenicity: FT0 amplitude bug, multiplicity classes, code cleanup

[PaolaVT]

No description provided.

[github-actions]

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

[alibuild]

Error while checking build/O2Physics/code-check for 9a5a550 at 2026-08-17 00:07:

++ set -euo pipefail
++ [[ -d /sw/slc9_x86-64/Clang/v20.1.7-24 ]]
+++ realpath /sw/BUILD/22e6c3b41e1fea4c97de34a6ebd935bc5234d960/O2Physics-code-check/../../O2Physics-latest/O2Physics
++ O2PHYSICS_BUILDDIR=/sw/BUILD/4de5872e63a714e4a96d2b18a13d963ddb37a78b/O2Physics
++ cp /sw/BUILD/4de5872e63a714e4a96d2b18a13d963ddb37a78b/O2Physics/compile_commands.json .
+++ python3 -c 'import json, os; print(os.path.commonpath([x["file"] for x in json.loads(open("compile_commands.json").read()) if "sw/BUILD" not in x["file"] and "G__" not in x["file"] and x["file"].endswith(".cxx")]))'
++ O2PHYSICS_SRC=/sw/SOURCES/O2Physics/slc9_x86-64-slc9_x86-64/0
++ pushd /sw/SOURCES/O2Physics/slc9_x86-64-slc9_x86-64/0
/sw/SPECS/slc9_x86-64/O2Physics-code-check/1.0-local644/O2Physics-code-check.sh: line 16: pushd: /sw/SOURCES/O2Physics/slc9_x86-64-slc9_x86-64/0: No such file or directory

Full log here.