Proposal for bipartite alltoall collective channel.

Rationale:
Many applications need to reorganize a distributed matrix from one group of processes to another. 
This is the second of two alltoall channels that support that. This alltoall channel redistributes 
data from one group (sender) to another one (receiver). The two groups may share processes. Many 
platform specific features can be used to optimize this channel beyond what users can do themselves 
as well as making more performance portable for the users.


1. Changes to section 8.
Add 
8.8 Bipartite Alltoall Operation Channel
`
This section defines operations for creating and manipulating a bipartite alltoall collective channel. 
The alltoall operation is analogous to the scatter & gather performed together. Each process that 
belongs to the sender group scatters its message among processes from the receiver group of the 
alltoall channel. The processes of the receiver group then gathers all the incoming messages from all 
sender group processes into a single buffer. The send message buffer is evenly divided into small 
messages with the number of small messages equal to the size of the receiver group. Each small message 
is sent to the corresponding process of the receiver group with the i-th receiver process gets the 
i-th small sender message. The arriving small messages are then concatenated into a single buffer that 
is inserted by an implementation into a recvoutbufiter. The order of dividing a send message into 
smaller messages for individual receiver processes follows the order of the processes in the receiving 
group. The order of concatenation of receiving small messages from the sender processes into an 
alltoall channel receive message follows the ordering of the processes of the channel group. All send 
buffers at the sender processes are of the same size and all recv buffers at the receiver processes 
are of the same size. Both sizes are divisible by the cardinality of the sender and receiver groups 
and dataspec size.

MPIRT_BIALLTOALL_CHANNEL_CREATE (name, inbufiter, outbufiter, recvinbuffer, recvoutbufiter, group, 
                                receiver_group, qos, bialltoallchannel)
IN              name            name of the collective channel (MPIRT_STRING_NAME)
IN              inbufiter       sender side input buffer iterator that is significant only for the sender 
                                group and MPIRT_BUFITER_NULL for the receiver processes (MPIRT_BUFITER)
IN              outbufiter      sender side output buffer iterator that is significant only for the sender 
                                group and MPIRT_BUFITER_NULL for the receiver processes (MPIRT_BUFITER)
IN              recvinbufiter   receiver side input buffer iterator that is significant only for the 
                                receiver group and MPIRT_BUFITER_NULL for the sender processes (MPIRT_BUFITER)
IN              recvoutbufiter  receiver side output buffer iterator that is significant only for the 
                                receiver group and MPIRT_BUFITER_NULL for the sender processes (MPIRT_BUFITER)
IN              group           group whose processes send messages (MPIRT_GROUP)
IN              receiver_group  group whose processes receive messages (MPIRT_GROUP)
IN              qos             channel quality of service (MPIRT_QOS_name-of-QOS-CHANNEL-class)
OUT             bialltoallchannel    bipartite alltoall collective channel (ref. to MPIRT_BIALLTOALL_CHANNEL)


[C and C++ will be added here.]

[Two pair of set/get operations for recvinbufiter and recvoutbufiter will be added here.
A pair of set/get operations for receive_group will be added here. The set/get operations for sender group 
are provided in chapter 6.]

[The tables Miscellaneous definitions and bindings will be changed to add bipartite alltoall channel. One more 
table for bipartite alltoall channel will be added at the end.]

2. [Changes to Chapter 2.]
Add BIALLTOALL_CHANNEL to table 2.1 (page 21) for meta classes name-of-class, name-of-class-for-free-dup, 
name-of-class-for-attributes, name-of-class-for-comparison, name-of-leaf-class, name-of-leaf-class-for-create 
and name-of-channel-class.

Add Bipartite_Alltoall_Channel to figure 2.1 on page 22.

3. [Changes to Chapters 6 & 9.]
Add Bipartite_Alltoall channel to first footnote on page 117 and the sentence on page 119 lines 28-31. Add 
Bipartite_Alltoall channel to the sentence on page 162 lines 38-42.

4. [Changes to the indexes]
Bipartite Alltoall channel and its functions should appear in the function and entities indexes at the end of 
the document.


[Alltoall channel is consistent with the variable buffer proposal.]