Write a program to measure the time it takes to send 1, 2, 4, ..., 1M C
doubles from one processor to another using MPI_Isend, MPI_Irecv, and
MPI_Wait.  Use the same techniques as in the memcpy assignment to average out
variations and overhead in MPI_Wtime.
<P>
Print the size, time, and rate in MB/sec for each test.
<P>
Make sure that both sender and reciever are ready when you begin the test.
The sample solution uses MPI_Sendrecv, but other choices are possible.
Compare the performance to that when using MPI_Send and MPI_Recv.
