Support external hit-vector indexing.

[osschar]

This still works with existing code.

Example of usage is shown in EventOfHits::SuckInHits(), commented out.

Hit remapping to external / CMSSW indices is shown in MkBuilder::remap_track_hits(..), also commented out.
Some more comments from there:
// Note that the indices after this are correct CMSSW "large-vector" indices.
// So no hit/layer mapping code in CMSSW producer is needed.
//
// We could really store original indices into HitOnTrack.index
// from the start. One just needs to be careful when getting hits in backwards fit,
// to use (a currently non-existent) LayerOfHits::GetHitWithOriginalIndex(hot.index)
//
// Further, somebody should walk over quite a bit of validation code that is
// rather involved doing such remappings.

[makortel]

Just to note here explicitly that by visual inspection the API to add hits and extract hits of an mkfit-track look ok from CMSSW perspective. The only missing piece is the call to runBuildingTestPlex*() with only EventOfHits + TrackVec + hit mask, which will be looked more after @mmasciov's upcoming changes.

[makortel]

Should we consider merging this PR?