Handle CAF making when Pandora runs after the NuGraph2 filter

[rtriozzi]

This PR, based on work by summer intern Leonardo Lena, adds a mode governed by the UsePandoraAfterNuGraph FHiCL flag to handle NuGraph2 variables in the CAF maker, when using a second-pass Pandora after filtering noise with NuGraph2. Relevant information about the NuGraph2 chain is provided in icaruscode PR #868.

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Associated PRs

• Introduce multi-slice NuGraph2 inference and filtering icaruscode#868

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Review

Tagging for review @acampani and @PetrilloAtWork as reconstruction and infrastructure experts. Thanks a lot!

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Description

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[cerati]

Thanks, overall it looks great -- especially removing the need for the hit mapping makes things much simpler.

The two things I would suggest to change are:

1. Avoid erasing elements from vectors. I think it should be much easier to simply add a check that the element is non-null in the Fill*NuGraph method. Or is there another reason for erasing the elements with null nugraph score?
2. There is no need to get the NuGraph slices from the event if then we just use slices (when UsePandoraAfterNuGraph=true)