Generalize the monitor class

[Thoemi09]

• added a duplicate and a free function to the mpi::communicator class
• generalized the mpi::monitor class to handle general events instead of errors
  • renamed member functions
  • allow to check if at least one event has occurred on some process (some_event_occurred()) or if an event has occurred on all processes (all_events_occurred())
  • use the given communicator instead of MPI_COMM_WORLD
  • make root a member variable which defaults to 0

[Wentzell]

Thank you @Thoemi09 for this generalization and for incorporating the first round of feedback we discussed. See additional suggestions below.