Improve TimeTasks

[alecjohnson]

Issue #17 introduced the TimeTasks class to implement basic profiling.

A simpler and more general approach is possible based on macros and automatic destructors.

Create a set of time_task macros to be used as follows:

  void mymethod()
  {
    // record time from this point to the end of this scope.
    timeTasks_set_main_task(Task::PARTICLES);
    ...
    // block that does communication
    {
        // mode is reset to communication until end of scope
        timeTasks_set_communicating();
        ...
    }
  }

Improvements:

• use of destructors: The implementation of the time_task macros constructs a non-anonymous class instance whose destructor is called when it goes out of scope. This eliminates the need to make separate calls to start() and end() methods.
• defining subtasks is now supported.