Improve the alpaka-based prefix scan

[fwyzard]

Currently the logic used by the prefix scan is scattered and replicated in many files, e.g.

• HeterogeneousCore/AlpakaInterface/interface/prefixScan.h (obviously)
• RecoLocalTracker/SiPixelClusterizer/plugins/alpaka/ClusterChargeCut.h
• RecoLocalTracker/SiPixelClusterizer/plugins/alpaka/SiPixelRawToClusterKernel.dev.cc
• etc.

We should implement a single prefixscan(acc, ...) function that is able to deal with arbitrary sized inputs and automatically splits the work in multiple passes, as needed.

It should also use the compile-time warp size (if available), and fall back to a simple loop for single-threaded back-ends.

[fwyzard]

assign heterogeneous

[cmsbuild]

New categories assigned: heterogeneous

@fwyzard,@makortel you have been requested to review this Pull request/Issue and eventually sign? Thanks

[cmsbuild]

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[cmsbuild]

A new Issue was created by @fwyzard.

@Dr15Jones, @antoniovilela, @makortel, @mandrenguyen, @rappoccio, @sextonkennedy, @smuzaffar can you please review it and eventually sign/assign? Thanks.

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