Remove using-directives from common headers

[vkucera]

Using-directives should never appear in headers because they irreversibly pollute the global namespace wherever they are included and can lead to unexpected behaviour, such as name conflicts. See O2 CodingGuidelines.

In fact, the evsel namespace was indeed defined ambiguously, once as o2::aod::evsel in EventSelection.h and another time as just evsel in EventSelectionParams.h. Maybe @ekryshen can comment whether this was intentional or not.

@ddobrigk I think we should have a CI check that forbids using namespace in header files. That would require fixing remaining occurrences in the PWG directories (which I did not do in this PR).

[vkucera]

Sorry, there were a few fixes needed. It should be ready for merging now.

[aalkin]

Regarding evsel and aod::evsel, I would prefer them separate. Namespace aod is reserved for columns and tables. Probably the functional namespace can be something like evsel_impl or evsel_detail, since it is supposed to be distinct from aod::evsel which contains column definitions.

[vkucera]

Hi @aalkin , ok. Are you fine with renaming in a separate PR?

[aalkin]

@vkucera yes, of course, let's not waste the approvals :)

[vkucera]

@ddobrigk @TimoWilken Is it fine for you to merge this without waiting for the remaining approvals?

[ddobrigk]

To avoid any conflicts, maybe we can manual-force-merge? @ktf or @pzhristov maybe? Thanks a lot!

[alibuild]

Error while checking build/O2Physics/o2 for 9805abf at 2023-08-16 17:53:

## sw/BUILD/O2Physics-latest/log
[ERROR] Function RecoDecay::getMassPDG is deprecated and will be removed soon.
[ERROR] Please use the Mass function in the O2DatabasePDG service instead.
[ERROR] See the example of usage in Tutorials/src/usingPDGService.cxx.
[ERROR] Function RecoDecay::getMassPDG is deprecated and will be removed soon.
[ERROR] Please use the Mass function in the O2DatabasePDG service instead.
[ERROR] See the example of usage in Tutorials/src/usingPDGService.cxx.
[ERROR] Function RecoDecay::getMassPDG is deprecated and will be removed soon.
[ERROR] Please use the Mass function in the O2DatabasePDG service instead.
[ERROR] See the example of usage in Tutorials/src/usingPDGService.cxx.
[ERROR] Function RecoDecay::getMassPDG is deprecated and will be removed soon.
[ERROR] Please use the Mass function in the O2DatabasePDG service instead.
[ERROR] See the example of usage in Tutorials/src/usingPDGService.cxx.
[ERROR] Function RecoDecay::getMassPDG is deprecated and will be removed soon.
[ERROR] Please use the Mass function in the O2DatabasePDG service instead.
[ERROR] See the example of usage in Tutorials/src/usingPDGService.cxx.
[ERROR] Function RecoDecay::getMassPDG is deprecated and will be removed soon.
[ERROR] Please use the Mass function in the O2DatabasePDG service instead.
[ERROR] See the example of usage in Tutorials/src/usingPDGService.cxx.
[ERROR] Function RecoDecay::getMassPDG is deprecated and will be removed soon.
[ERROR] Please use the Mass function in the O2DatabasePDG service instead.
[ERROR] See the example of usage in Tutorials/src/usingPDGService.cxx.
/sw/SOURCES/O2Physics/3324-slc7_x86-64/0/PWGHF/TableProducer/treeCreatorBsToDsPi.cxx:358:24: error: 'array' was not declared in this scope; did you mean 'std::array'?
ninja: build stopped: subcommand failed.

Full log here.

[ktf]

Merged this now. I have noticed two other quite bad practices, while reviewing this:

• For no reason one should have #include <iostream> in headers, as it bloats every single file which includes it with statics.
• In general there should not be any reason for having statics in the header files.