Variable gather operations

Gathers data if varying size to a single process.

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

int main() {
  const mpl::communicator &comm_world{mpl::environment::comm_world()};
  const auto c_rank{comm_world.rank()};
  const auto c_size{comm_world.size()};
  // gather a single int from all ranks to rank root=0
  {
    int root{0};
    int x{c_rank + 1};
    std::vector<int> y(c_rank == root ? c_size : 0);
    comm_world.gather(root, x, y.data());
    if (c_rank == root) {
      for (int i{0}; i < c_size; ++i)
        std::cout << y[i] << ' ';
      std::cout << "\n";
    }
  }
  // gather a single int from all ranks to rank root=0
  // root and non-root rank use different function overloads of gather
  {
    const int root{0};
    int x{-(c_rank + 1)};
    if (c_rank == root) {
      std::vector<int> y(c_size);
      comm_world.gather(root, x, y.data());
      for (int i{0}; i < c_size; ++i)
        std::cout << y[i] << ' ';
      std::cout << "\n";
    } else
      comm_world.gather(root, x);
  }
  // gather several ints from all ranks to rank root=0
  {
    const int root{0}, n{3};
    std::vector<int> x(n, c_rank + 1);
    std::vector<int> y(c_rank == root ? n * c_size : 0);
    mpl::contiguous_layout<int> l(n);
    comm_world.gather(root, x.data(), l, y.data(), l);
    if (c_rank == root) {
      for (int i{0}; i < c_size * n; ++i)
        std::cout << y[i] << ' ';
      std::cout << "\n";
    }
  }
  return EXIT_SUCCESS;
}