Fix mpi gather scatter

This MR fixes the wrong computation of the data length in igather and iscatter. In addition, it adds a test that uses igather and iscatter with a datatype that has length larger than 1.

dune/common/parallel/test/CMakeLists.txt

@@ -51,3 +51,10 @@ dune_add_test(SOURCES mpipacktest.cc
               TIMEOUT 300
               CMAKE_GUARD MPI_FOUND
               LABELS quick)
+
+dune_add_test(SOURCES mpigatherscattertest.cc
+              LINK_LIBRARIES dunecommon
+              MPI_RANKS 2
+              TIMEOUT 300
+              CMAKE_GUARD MPI_FOUND
+              LABELS quick)