In #136 I couldn't get `PetscCommDestroy` and `PetscCommDuplicate` to work to increase PETSc refcount on the communicator. See [here](https://github.com/JuliaParallel/PETSc.jl/blob/827e7e195910a04712e8f793fddb718fb4582dd2/src/sys.jl#L19-L58)
