New SoftMuon ID selector

Hi, 
this is an update of the SoftMuon ID selector in
DataFormats/MuonReco/src/MuonSelectors.cc 
as described in the Muon POG Twiki: 
https://twiki.cern.ch/twiki/bin/view/CMSPublic/SWGuideMuonId#New_Version_recommended. 
The optimization of the new working point was carried out by I. Krätschmer, as well as the performance studies: 
https://indico.cern.ch/event/277448/material/slides/0?contribId=3
https://indico.cern.ch/event/316122/contribution/3/material/slides/0.pdf. 
In few words, the cut on the number of pixel layers is loosened from >1 to >0, the transverse and longitudinal impact parameter cuts are tightened from 3cm to 0.3cm and from 30cm to 20cm, respectively, and the cut on the track chi^2 is replaced by a requirement for the track to be high-purity. 
The change does not have any effect at reconstruction level.