some of the collective modules. Added a new function
opan_datatype_span, to compute the memory span of
count number of datatype, excluding the gaps in the
beginning and at the end. If a memory allocation is
made using the returned value, the gap (also returned)
should be removed from the allocated pointer.
in the base).
Correctly deal with persistent requests (they must be always freed when
they are stored in the request array associated with the communicator).
Always use MPI_STATUS_IGNORE for single request waiting functions.
