Changed best HS vertex selection to use number of HS tracks
This MR updates the selection of the best matched reco HS vertex to use the number of HS tracks included, instead of the HS track purity. This criteria was found to be more sensible from Upgrade studies https://indico.cern.ch/event/952070/contributions/4102860/attachments/2141189/3607896/AQT_219comp.pdf (slide 4) and matches what was used for 20.20 upgrade studies.