mpi: first draft for #12

- a thin layer, using a bit boost::mpi (for the communicator mostly ...)
  along the lines discussed in #12.
- implemented reduce, allreduce, bcast for arrays, simple scalars,
and any custom type that support boost serialization.
- Custom types :  the operations are done recursively on members.
  No change is needed in the class to use this mpi routine, as long as
  serialize function is defined.
- For arrays of basic types (int, double...), a direct call to MPI C API, which works also for views
  (as long as they are contiguous).
- For arrays of more complex types, we revert to boost::mpi.
- Added a simple test.
- Work still in progress :
   - missing a simple scatter/gather for the arrays
   - need more tests & API thinking.
   - dispatch array code to array lib
   - reduce is "sum" only, but do we need more.