Create() does not work MPI-parallel with arrays of parameter values

The following should work

```bash
mpirun -np 2 python -c 'import nest; nest.Create("iaf_psc_alpha", 4, {"V_m": [-77.0, -66.0, -77.0, -66.0], "tau_m": [10.0, 11.0, 12.0, 13.0]})'
```

but causes strange type errors both with master and pynest-ng. It works when run with just a single process. I suspect the problem lies in the Python interface that somehow manipulates the dictionary content.