ENH: adjust infoSwitch to report host subscription (related to #531)

- this compact form shows the subscription per host in the unsorted
  mpi order

      nProcs : 18
      Hosts  :
      (
          (node1 6)
          (node2 8)
          (node3 4)
      )

  This provides a succinct overview of which hosts have been
  subscribed or oversubscribed.

- The longer list of "slave.pid" ... remains available on the
  InfoSwitch 'writeHosts'