ITSMFT: scaffolding for staggered digitization

[f3sch]

This adds the possibility to 'stagger' the ITS/MFT detector digitisation. Currently this is disabled and steered by the a flag in the DPLAlpideParam. The output e.g. Branch names, digits themselves does not change from before.

[github-actions]

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[sawenzel]

Could you please add few more information what this achieves? How and where is this useful?

[f3sch]

The basic idea, is that the digit output is per layer and not anymore in a flat array. Then one can define per layer differing readout frame lengths, biases and offsets. The final goal is to also adapt everything up to the tracking to handle different ROFs, which is needed for ITS3 but already in ITS2 maybe tested (actually this data has already been taken).

[sawenzel]

Ok. Fine from my side as long as it's optional and not affecting production.