add mpi hello world

hpc/hello-world/mpi_hello.c

+#include <mpi.h>
+#include <stdio.h>
+
+int main(int argc, char** argv) {
+    // Initialize the MPI environment
+    MPI_Init(NULL, NULL);
+
+    // Get the number of processes
+    int world_size;
+    MPI_Comm_size(MPI_COMM_WORLD, &world_size);
+
+    // Get the rank of the process
+    int world_rank;
+    MPI_Comm_rank(MPI_COMM_WORLD, &world_rank);
+
+    // Get the name of the processor
+    char processor_name[MPI_MAX_PROCESSOR_NAME];
+    int name_len;
+    MPI_Get_processor_name(processor_name, &name_len);
+
+    // Print off a hello world message
+    printf("Hello world from processor %s, rank %d"
+           " out of %d processors\n",
+           processor_name, world_rank, world_size);
+
+    // Finalize the MPI environment.
+    MPI_Finalize();
+}
+