DerivationFrameworkTau: add truth variables to smart slimming list only when running over MC

Hello,

As discussed in ATLASRECTS-6902, when running AOD->DAOD for data, there are many warnings about non-existing variables. These warnings are coming from CP smart slimming lists that provide the same list of variables for data and MC, while some variables only exist in MC. This MR implements a proposal to provide MC-only variables only when running over MC AODs. I checked that the DAOD content is unchanged, both for data and MC. The only warnings that disappear when running over are:

AODtoDAOD 13:38:29 xAOD::AuxSelection WARNING Selected dynamic Aux atribute "truthMatchProbability" not found in the registry
AODtoDAOD 13:38:29 xAOD::AuxSelection WARNING Selected dynamic Aux atribute "truthJetLink" not found in the registry

So it's only a small improvement, there are 37 remaining warnings coming from other domains...

Cheers, Bertrand