Improving SL PWG selection introduced in !2981

Selection introduced in !2981 (merged) is inefficient and creating too many combinations as listed at !2981 (comment 7947554):

GaudiTest.lines.WARNING	
  70 - SbCombiner_onetrack_1_SpruceSLB_... WARNING Suppressing message: 'Too many composites. Stopping the combination.'
  71 - SbCombiner_onetrack_2_SpruceSLB_... WARNING Suppressing message: 'Too many composites. Stopping the combination.'
  72 - SbCombiner_onetrack_2_SpruceSLB_... WARNING Suppressing message: 'Too many composites. Stopping the combination.'

Selection has to be improved in future to reduce number of combinations selected by the SL combiners.