MPI is added for actual communication.
Tasks are correctly sent and received by processors.
DuctTeip engine is modified to be able to:
  - send tasks via Mailbox
  - receive tasks from MailBox
  - create work items (currently for tasks)
  - execute work items(currently for tasks)
  - manage work items in a queue (FIFO)
  - process the state_machine of objects by work items.
Correctness test passed.
The program (test8) runs forever due to an infinite loop for processing
mailbox events and/or work items. Thus it SHOULD BE KILLED manually.
The output trace of program grows rapidly.