tauRec and tauRecTools: add dynamic variable to flag thinned taus

Hello,

In R22, we run a thinning algorithm before writing tau containers to AOD. Taus below a configurable pt threshold are not written to AOD. So far, tau thinning was applied in the ESDtoAOD step, after the actual tau reconstruction that runs in the RAWtoESD step. But this is inconvenient for client codes that need to know at reconstruction time whether a tau is worth processing.

This MR changes the tau thinning strategy. A new dynamic variable is added onto the tau by a dedicated tool that runs in RAWtoESD. This variable can later be retrieved by client code which can then skip the taus that will be thinned. The variable is removed from the AOD content.

This should help mitigate the CPU increase reported in ATLASRECTS-6412, as well as the problem with invalid element links reported in ATLPHYSVAL-771, once PFTauFlowElementAssoc uses the newly added variable.

The MR does not change the tau reconstruction output and was tested with q221 on 200 events.

Adding the 'urgent' flag as there are a couple of upcoming MRs that will rely on the tau thinning flag (needed for the R22 reprocessing).

Tagging @christos and @mhodgkin .

Also tagging @cdelitzs , @mswiatlo and @wbalunas in case this is to be used for the MET tau term.

Cheers, Bertrand