WIP: Starting working on new tracking modes: DisplacedSoftPionSLHC and SLHCPixelPrdAssociation

I'm starting including the DisplacedSoftPionSLHC and SLHCPixelPrdAssociation configuration cuts. This is based on what was implemented time ago as an R&D project from the unconventional tracking TF exploring the possibility to reconstruct low momentum pions (pT~300 MeV) originating from long-lived charged particles. The old merge request is here: https://gitlab.cern.ch/atlas/athenaprivate1/-/merge_requests/13402

Some of the tools and algorithms are already available. They need updates for the ITk use-case.

As far as I see the needed updates are related to:

• Implement DisplacedSoftPionSLHC and SLHCPixelPrdAssociation tracking mode. The former runs after the SLHCPixelPrdAssociation configuration, since the seeding algorithm mentioned below uses tracks from SLHCPixelPrdAssociation as input to define the RoI.
• Provide track seeding algorithm for ITk using the RoITool.
• Use VrtSecDecay: this algorithm takes the SLHCPixelPrdAssociation and tracks from the DisplacedSoftPionSLHC tracking as input and fits for the two track vertex. Sanity and selection cuts need to be defined for the HL-LHC scenario.

I start porting the first version of the code where the machinery is in place to use the new tracking configurations, while configuring the ITk tools for track seeding and track finding.

I'm currently working on adapting (if needed) the RoITool and provide the track seeding algorithm for ITk.

FYI: @pagessin @amorley @asalzbur @nstyles @tstreble @npetters @goblirsc

Comments and advices are very welcome!