Blocking send and receive

Demonstrates various blocking send and receive modes of standard data types.

#include <cstdlib>
#include <iostream>
#include <mpl/mpl.hpp>

int main() {
  const mpl::communicator &comm_world{mpl::environment::comm_world()};
  // run the program with two or more processes
  if (comm_world.size() < 2)
    return EXIT_FAILURE;
  // process 0 sends
  enum class tag_enum : short { tag = 1 };
  if (comm_world.rank() == 0) {
    // see MPI Standard for the semantics of standard send, buffered send,
    // synchronous send and ready send
    double x{1.23456};
    comm_world.send(x, 1, tag_enum::tag);  // send x to rank 1 via standard send
    ++x;
    {
      // create a buffer for buffered send,
      // memory will be freed on leaving the scope
      int size{comm_world.bsend_size<decltype(x)>()};
      mpl::bsend_buffer buff{size};
      comm_world.bsend(x, 1);  // send x to rank 1 via buffered send
    }
    ++x;
    comm_world.ssend(x, 1);  // send x to rank 1 via synchronous send
    ++x;
    comm_world.rsend(x, 1);  // send x to rank 1 via ready send
  }
  // process 1 receives
  if (comm_world.rank() == 1) {
    double x;
    comm_world.recv(x, 0, tag_enum::tag);  // receive x from rank 0
    std::cout << "x = " << x << '\n';
    comm_world.recv(x, 0);  // receive x from rank 0
    std::cout << "x = " << x << '\n';
    comm_world.recv(x, 0);  // receive x from rank 0
    std::cout << "x = " << x << '\n';
    comm_world.recv(x, 0);  // receive x from rank 0
    std::cout << "x = " << x << '\n';
  }
  return EXIT_SUCCESS;
}