[bugfix] Make sure MPI_FInalize is called before the return in main.

A correct MPI program should do that.

As MPI_Finalize is part of the destructor of MainObject we need to
make sure that it's destructor is called before the return statement.
We do that manually  by resetting the unique_ptr that we now use to
store the MainObject,

flow/flow_ebos_onephase.cpp

@@ -84,8 +84,11 @@ int flowEbosWaterOnlyMain(int argc, char** argv, bool outputCout, bool outputFil
 int flowEbosWaterOnlyMainStandalone(int argc, char** argv)
 {
     using TypeTag = Opm::Properties::TTag::EclFlowProblemWaterOnly;
-    auto mainObject = Opm::Main(argc, argv);
-    return mainObject.runStatic<TypeTag>();
+    auto mainObject = std::make_unique<Opm::Main>(argc, argv);
+    auto ret = mainObject->runStatic<TypeTag>();
+    // Destruct mainObject as the destructor calls MPI_Finalize!
+    mainObject.reset();
+    return ret;
 }
 
 }