Document and clean PBclust.display_clust_report
Document and clean PBclust.display_clust_report

Add a docstring to the `display_clust_report` function, and simplify the
function by using collections.Counter rather than reimplementing the
counting.

HubLot/PBxplore,jbarnoud/PBxplore,jbarnoud/PBxplore,pierrepo/PBxplore,HubLot/PBxplore,pierrepo/PBxplore