Molloy Reed model not allways correct

The Molloy Reed model does not always give the correct network back.
This leads to an unstable test of the `mean_degree` which is currently using a Molloy Reed model for testing.

```python
def test_mean_degree():
    """Test the mean degree calculation."""
    net = pp.generators.Molloy_Reed([2]*500)
    assert sum(net.degrees().values()) == 1000
```