TauAnalysisTools: clean up truth matching tools

Hello,

This MR is cleaning up the tau truth matching tools. The code behaviour is almost unchanged, with the exception of the TauTruthMatchingTool. Currently, in case the reco tau is matched to a true tau, or a true electron, or a true muon, we decorate the reco tau with an element link called "truthParticleLink". And in addition, for reco taus matched to true taus, we add a second element link called "TruthTau" (misleading name as it's an element link, and redundant). I don't think we use the latter anywhere in TauAnalysisTools, so I dropped it. In case it's of any use, we can resurrect "TruthTau" via a helper function that would use "truthParticleLink".

Tagging @ghamity and @ademaria for a few more pairs of eyes.

Cheers, Bertrand