ITS: simplify configuration of iterations

[f3sch]

Simplifies configuration of individual iterations (should make it easier to add more configurations in the future), especially attacking this kind of pattern with hardcoded iterations:

if (iteration == 0 || iteration == 3) {
    prepareClusters(trkParam, maxLayers);
  }

[f3sch]

@fprino, @shahor02 what do you think about this?

[shahor02]

@f3sch the idea is good, but from the superficial check cannot say I understood all details (even leaving aside the gpu part) :)
Will try to check in more details later.

[alibuild]

Error while checking build/O2/fullCI_slc9 for 29435a5 at 2026-05-08 06:18:

## sw/BUILD/O2-latest/log
/sw/SOURCES/O2/slc9_x86-64-slc9_x86-64/0/Detectors/ITSMFT/ITS/tracking/GPU/cuda/TrackingKernels.cu(1263): error: no instance of function template "o2::its::countTrackletsInROFsHandler" matches the specified type
/sw/SOURCES/O2/slc9_x86-64-slc9_x86-64/0/Detectors/ITSMFT/ITS/tracking/GPU/cuda/TrackingKernels.cu(1290): error: no instance of function template "o2::its::computeTrackletsInROFsHandler" matches the specified type
ninja: build stopped: subcommand failed.

Full log here.

[alibuild]

Error while checking build/O2/fullCI_slc9 for c752ef1 at 2026-05-10 09:30:

## sw/BUILD/O2-latest/log
collect2: error: ld returned 1 exit status
ninja: build stopped: subcommand failed.

Full log here.

[f3sch]

Maybe for context for the GPU-parts, we have in many functions where we transfer data to the device, this call structure:

void loadStuff(int iteration){
   if (!iteration) {
     // ....
   }
}

which I moved simply into a more general if in TrackerTraitsGPU:

void initialise(int iteration){
   if(mTrkParams[iteration][LoadStuff]){
      loadStuff1();
      loadStuff2();
      //....    
   }
}

so the actual change is negligible and trivial but it looks like a lot since the call signatures get modified.

[shahor02]

@f3sch thanks for the hints! As far as I could see, all is fine.

[f3sch]

@njacazio @fcolamar please approve