This exercise presents a simple program to determine the value of pi. 
The algorithm suggested here is chosen for its simplicity.  The
method evaluates the integral of 4/(1+x*x) between -1/2 and 1/2.
The method is simple: the integral is approximated by a sum of n intervals;
the approximation to the integral in each interval is (1/n)*4/(1+x*x).
The master process (rank 0) asks the user for the number of intervals;
the master should then broadcast this number to all of the other processes.
Each process then adds up every n'th interval (x = -1/2+rank/n, 
-1/2+rank/n+size/n,...).  Finally, the sums computed by each process are 
added together using a reduction.

