cheap parallel prime number tester.
region division with remainder(calculate istart and iend for each process) could be useful for some other cases...?

$ make

$ time ./seri 2147483647
val: 2147483647
this is a prime number.

real 0m11.653s
user 0m11.641s
sys 0m0.001s

$ time ./omp 2147483647
val: 2147483647
this is a prime number.

real 0m1.124s
user 0m14.925s
sys 0m0.258s

$ time srun --mpi=pmi2 -n64 -c1 --cpu_bind=cores -m block:block ./mpi 2147483647
 val:            2147483647
 this is a prime number.

real    0m0.897s
user    0m0.011s
sys     0m0.015s


CPU: Intel(R) Xeon(R) CPU E5-2680 0 @ 2.70GHz, 2 sockets/8 cores per socket
MPI version: 4 nodes, 64 cores, intelmpi 2018u0