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.