Work around MPI startup race condition on macOS

If a process started by mpiexec finishes too quickly, sometimes some of the other processes are not started.
It is unclear whether this is an OpenMPI or macOS bug, but appears significantly more often since moving from macOS 10.13 to 10.15. OpenMPI 4.0.1 and 3.0.0 both show the issue on macOS 10.15.

src/core/mpi/MPIManager.cpp

@@ -32,6 +32,9 @@
 #include <stdexcept>
 #include <string>
 #include <vector>
+#ifdef __APPLE__
+#include <thread>
+#endif
 
 namespace walberla
 {
@@ -94,6 +97,12 @@ void MPIManager::initializeMPI(int* argc, char*** argv, bool abortOnException)
       MPI_Initialized(&mpiAlreadyInitialized);
       if (!mpiAlreadyInitialized)
       {
+#ifdef __APPLE__
+         /* Work around a race condition on macOS.
+            If a process started by mpiexec finishes too quickly, it sometimes doesn't start all processes.
+          */
+         std::this_thread::sleep_for(std::chrono::milliseconds(1000));
+#endif
          MPI_Init(argc, argv);
          finalizeOnDestruction_ = true;
       }