tauRec: keep TRT-only tau tracks thinned by egammaTrackSlimmer

Hello,

This MR is implementing a functionality to keep ID tracks associated with taus that are thinned by egammaTrackSlimmer (currently breaking the tau track -> ID track navigation in xAODs). Relevant ticket: ATLTAU-1770. This track recovery is only triggered if egammaTrackSlimmer is scheduled (which is true by default). It introduces an ad-hoc dependency on the egammaTrackSlimmer code, as we must match exactly the thinning condition, but I see no other way to proceed.

Tagging egamma expert @christos . Could you please check if the implementation looks fine?

Cheers, Bertrand