Extend multi process tutorial

[njacazio]

No description provided.

[alibuild]

Error while checking build/O2Physics/o2 for 91b067b at 2022-08-19 20:35:

## sw/BUILD/O2Physics-latest/log
/sw/SOURCES/O2Physics/1149-slc7_x86-64/0/Tutorials/src/multiProcess.cxx:49:7: error: 'histos' was not declared in this scope
/sw/SOURCES/O2Physics/1149-slc7_x86-64/0/Tutorials/src/multiProcess.cxx:54:7: error: 'histos' was not declared in this scope
/sw/SOURCES/O2Physics/1149-slc7_x86-64/0/Tutorials/src/multiProcess.cxx:59:7: error: 'histos' was not declared in this scope
ninja: build stopped: subcommand failed.

Full log here.