[PWGEM] Store the mixing categories if flag is true

[amaringarcia]

No description provided.

[github-actions]

O2 linter results: ❌ 0 errors, ⚠️ 239 warnings, 🔕 7 disabled

[alibuild]

Error while checking build/O2Physics/code-check for 272ec9b at 2026-09-29 21:41:

## sw/BUILD/O2Physics-code-check-latest/log
--
========== List of issues found ==========
++ echo 'Found 1 errors and 0 warnings.'
Found 1 errors and 0 warnings.
++ [[ 1 -gt 0 ]]
++ cat /sw/BUILD/f61c168600290dad66b25600bfed74e1a8f6783f/O2Physics-code-check/errors.txt
PWGEM/PhotonMeson/Utils/NMHistograms.h:38:42: error: no header providing "M_PI" is directly included [misc-include-cleaner,-warnings-as-errors]
++ [[ 0 -gt 0 ]]
++ [[ 1 -gt 0 ]]
++ exit 1
--

Full log here.

[mhemmer-cern]

Hi Ana, thank you for the PR.
Regarding the code-check error, could you please use the pi and fractions of pi from the common constants defined here:
https://github.com/AliceO2Group/AliceO2/blob/dev/Common/Constants/include/CommonConstants/MathConstants.h.

Regarding the formatting, please have a look at the pre-commit utility package, that can help automate formatting fixes locally before committing: https://aliceo2group.github.io/analysis-framework/docs/tools/#pre-commit-hooks