ITS: factor out CPU/GPU common code & separate Cell class

[f3sch]

Based on #15289 (including some commits there).

This reduces the total memory consumed by some percent by splitting the CellSeed class into a CellSeed (with only three cluster ind.) and a TrackSeed (with NLayers cluster ind.):

Additionally, this factors out common functions used by the GPU/CPU code (and actually fixes some GPU stuff) by creating a separate header with commonly used functions for the track/cell fit and the accounting of the final track output.

[alibuild]

Error while checking build/O2/fullCI_slc9 for 5cd39b1 at 2026-04-20 21:09:

## sw/BUILD/O2-latest/log
/sw/SOURCES/O2/slc9_x86-64-slc9_x86-64/0/Detectors/ITSMFT/ITS/tracking/include/ITStracking/TrackHelpers.h(56): error: calling a __device__ function("o2::track::TrackParametrizationWithError<float> ::resetCovariance(float)") from a __host__ __device__ function("resetTrackCovariance") is not allowed
/sw/SOURCES/O2/slc9_x86-64-slc9_x86-64/0/Detectors/ITSMFT/ITS/tracking/include/ITStracking/TrackHelpers.h(57): error: calling a __device__ function("_ZNK2o25track29TrackParametrizationWithErrorI1?E6getCovEv") from a __host__ __device__ function("resetTrackCovariance") is not allowed
/sw/SOURCES/O2/slc9_x86-64-slc9_x86-64/0/Detectors/ITSMFT/ITS/tracking/include/ITStracking/TrackHelpers.h(57): error: calling a __device__ function("_ZN2o25track29TrackParametrizationWithErrorI1?E6setCovE1?1?") from a __host__ __device__ function("resetTrackCovariance") is not allowed
/sw/SOURCES/O2/slc9_x86-64-slc9_x86-64/0/Detectors/ITSMFT/ITS/tracking/include/ITStracking/TrackHelpers.h(158): error: calling a __device__ function("_ZN2o25track29TrackParametrizationWithErrorI1?E6rotateE1?1?1?") from a __host__ __device__ function("fitTrack") is not allowed
/sw/SOURCES/O2/slc9_x86-64-slc9_x86-64/0/Detectors/ITSMFT/ITS/tracking/include/ITStracking/TrackHelpers.h(161): error: calling a __device__ function("_ZNK2o24base14PropagatorImplI1?E12propagateToXE1?1?1?1?1?1?1?1?1?") from a __host__ __device__ function("fitTrack") is not allowed
/sw/SOURCES/O2/slc9_x86-64-slc9_x86-64/0/Detectors/ITSMFT/ITS/tracking/include/ITStracking/TrackHelpers.h(168): error: calling a __device__ function("_ZN2o25track29TrackParametrizationWithErrorI1?E18correctForMaterialE1?1?1?1?") from a __host__ __device__ function("fitTrack") is not allowed
/sw/SOURCES/O2/slc9_x86-64-slc9_x86-64/0/Detectors/ITSMFT/ITS/tracking/include/ITStracking/TrackHelpers.h(173): error: calling a __device__ function("_ZN2o25track29TrackParametrizationWithErrorI1?E6rotateE1?") from a __host__ __device__ function("fitTrack") is not allowed
/sw/SOURCES/O2/slc9_x86-64-slc9_x86-64/0/Detectors/ITSMFT/ITS/tracking/include/ITStracking/TrackHelpers.h(176): error: calling a __device__ function("_ZNK2o24base14PropagatorImplI1?E12propagateToXE1?1?1?1?1?1?1?1?") from a __host__ __device__ function("fitTrack") is not allowed
/sw/SOURCES/O2/slc9_x86-64-slc9_x86-64/0/Detectors/ITSMFT/ITS/tracking/include/ITStracking/TrackHelpers.h(183): error: calling a __device__ function("_ZN2o25track29TrackParametrizationWithErrorI1?E18correctForMaterialE1?1?1?") from a __host__ __device__ function("fitTrack") is not allowed
/sw/SOURCES/O2/slc9_x86-64-slc9_x86-64/0/Detectors/ITSMFT/ITS/tracking/include/ITStracking/TrackHelpers.h(189): error: calling a __device__ function("_ZNK2o25track29TrackParametrizationWithErrorI1?E21getPredictedChi2QuietE1?1?") from a __host__ __device__ function("fitTrack") is not allowed
/sw/SOURCES/O2/slc9_x86-64-slc9_x86-64/0/Detectors/ITSMFT/ITS/tracking/include/ITStracking/TrackHelpers.h(194): error: calling a __device__ function("_ZN2o25track29TrackParametrizationWithErrorI1?E6updateE1?1?") from a __host__ __device__ function("fitTrack") is not allowed
/sw/SOURCES/O2/slc9_x86-64-slc9_x86-64/0/Detectors/ITSMFT/ITS/tracking/include/ITStracking/TrackHelpers.h(198): error: calling a __device__ function("_ZN2o25track20TrackParametrizationI1?E4setYE1?") from a __host__ __device__ function("fitTrack") is not allowed
/sw/SOURCES/O2/slc9_x86-64-slc9_x86-64/0/Detectors/ITSMFT/ITS/tracking/include/ITStracking/TrackHelpers.h(199): error: calling a __device__ function("_ZN2o25track20TrackParametrizationI1?E4setZE1?") from a __host__ __device__ function("fitTrack") is not allowed
ninja: build stopped: subcommand failed.

Full log here.