[BL/MPI.h] In / out buffers must not be aliased

See https://www.mpi-forum.org/docs/mpi-4.1/mpi41-report.pdf page 10

The following error message on JUWELS is fixed with this commit

vtkMPICommunicator.cxx:60    WARN| MPI had an error
------------------------------------------------
Invalid buffer pointer, error stack:
internal_Allgather(133): MPI_Allgather(sendbuf=0x1233940, sendcount=1, MPI_UNSIGNED_LONG, recvbuf=0x1233840, recvcount=1, MPI_UNSIGNED_LONG, MPI_COMM_WORLD) failed
internal_Allgather(85).: Buffers must not be aliased

BaseLib/MPI.h

@@ -101,10 +101,8 @@ static std::vector<T> allgather(T const& value, Mpi const& mpi)
 {
     std::vector<T> result(mpi.size);
 
-    result[mpi.rank] = value;
-
-    MPI_Allgather(&result[mpi.rank], 1, mpiType<T>(), result.data(), 1,
-                  mpiType<T>(), mpi.communicator);
+    MPI_Allgather(&value, 1, mpiType<T>(), result.data(), 1, mpiType<T>(),
+                  mpi.communicator);
 
     return result;
 }
@@ -116,10 +114,8 @@ static std::vector<T> allgather(std::vector<T> const& vector, Mpi const& mpi)
     // Flat in memory over all ranks;
     std::vector<T> result(mpi.size * size);
 
-    std::copy_n(vector.begin(), size, &result[mpi.rank * size]);
-
-    MPI_Allgather(&result[mpi.rank * size], size, mpiType<T>(), result.data(),
-                  size, mpiType<T>(), mpi.communicator);
+    MPI_Allgather(vector.data(), size, mpiType<T>(), result.data(), size,
+                  mpiType<T>(), mpi.communicator);
 
     return result;
 }