Template TrackListMerger and export copies for LHCb::Tracks and LHCb::Track::Selection

Olli Luptonrequested to merge
olupton_template_tracklistmerger into master

The existing TrackListMerger algorithm (which doesn't seem to be used anywhere) only accepted/produced LHCb::Track::Selection.

This MR turns it into a template and creates two instances, one using LHCb::Track::Selection as before (now called TrackSelectionMerger) and the other using LHCb::Tracks (called TrackListMerger).

This is useful for feeding the MuonID (which expects one input location) in ZombieMoore Hlt2 (which keeps long/downstream tracks in separate locations).

cc: @rmatev

Edited by Olli Lupton

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