TauTrig : improve choice of the lepton-tau pair to fill Tag and Probe chain monitoring

This MR solve an issue observed in the Tag and Probe chain monitoring in the 20 th validation round:

http://atlas-project-trigger-release-validation.web.cern.ch/atlas-project-trigger-release-validation/www/SampleA/ATR-25152-15-R3/r13501_vs_r13391/TauMon/TAndPVars/HLT_e26_lhtight_ivarloose_tau20_mediumRNN_tracktwoMVA_03dRAB_L1EM22VHI/hdR.html

about the choice of the lepton-tau pair chosen to fill the plots.

Issue has been discussed in https://indico.cern.ch/event/1143952/contributions/4805508/attachments/2416508/4135259/Trigger_T%26P_issue.pdf and we decided to choose a lepton-tau pair with dR(lep,tau) > 0.3 to avoid the issue of choosing overlapping tau and lepton.

Tagging @iriu and @gipezzul