[PWGCF] add multi-subevent support to FlowPtContainer

[EmilGorm]

• support for multiple subevent for pt-pt correlations (maximum of 4 to reduce output object size)
• backwards compatible
• flowGfwLightIons.cxx updated to use new features

[github-actions]

O2 linter results: ❌ 29 errors, ⚠️ 60 warnings, 🔕 4 disabled

[victor-gonzalez]

@EmilGorm at some point these linter errors need to be fixed

PWGCF/GenericFramework/Core/FlowPtContainer.h:42: error: Do not put using directives at global scope in headers. [using-directive]
Error: Do not put using directives at global scope in headers.
PWGCF/GenericFramework/Core/FlowPtContainer.h:43: error: Do not put using directives at global scope in headers. [using-directive]
Error: Do not put using directives at global scope in headers.