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,
this is used transferring ownership of setup structures to the
simulator. drop all the flowEbosXXX set deck methods and use the
generic vanguard. also means various structs that were only passed
in the blackoil simulator are now passed in all simulators.
