how to get constant density? broken?

What has become of:


```
        density:
            type : uniform
            value: 1.e-14 g/cm3
```

we want that to work e.g. for the comparison with syn++. But that type of density doesn't work now. @wkerzendorf @mklauser I assume this got lost (or the way we specify it changed?) as a result of the config validator changes?