[MPIManager] HAVE_MPI and libraries.
Throughout the code we try to put MPI statements only in header files. This is violated in _fem.cc where MPIManager::initialize is called.
Now the following could happen.
pip install dune-fem
Oh, I forgot PETSc. So
apt install petsc-dev
Then running a Python script where petsc is used as storage. Then PETSc is found but not correctly initialized because it's compiled into the _fem.so.