Add first mpi unit tests

tests/mpi_test_Messenger.cpp

 #include <catch.hpp>
 
 #include <Messenger.hpp>
+#include <Array.hpp>
+
+#include <pastis_config.hpp>
+
+#ifdef PASTIS_HAS_MPI
 #include <mpi.h>
+#define IF_MPI(INSTRUCTION) INSTRUCTION
+#else
+#define IF_MPI(INSTRUCTION)
+#endif // PASTIS_HAS_MPI
+
+namespace mpi_check
+{
+template <typename T>
+void test_allToAll()
+{
+  Array<T> data_array(commSize());
+  for (size_t i=0; i< data_array.size(); ++i) {
+    data_array[i] = commRank();
+  }
+  auto exchanged_array = allToAll(data_array);
+
+  for (size_t i=0; i< data_array.size(); ++i) {
+    REQUIRE(exchanged_array[i] == i);
+  }
+}
+}
 
 TEST_CASE("Messenger", "[mpi]") {
 
-  SECTION("check for parallel test") {
-    {
-      int rank;
-      MPI_Comm_rank(MPI_COMM_WORLD, &rank);
+  SECTION("communication info") {
+    int rank=0;
+    IF_MPI(MPI_Comm_rank(MPI_COMM_WORLD, &rank));
     REQUIRE(rank == commRank());
 
-      int size;
-      MPI_Comm_size(MPI_COMM_WORLD, &size);
+    int size=1;
+    IF_MPI(MPI_Comm_size(MPI_COMM_WORLD, &size));
     REQUIRE(size == commSize());
   }
+
+  SECTION("allToAll") {
+    mpi_check::test_allToAll<int8_t>();
   }
 }