NumberedObjectCollection append_renumber ignores negative numbers


**Describe the bug**

`NumberedObjectCollection.append_renumber` (e.g., `Cells.append_renumber`) considers `-1` a valid number. This is the placeholder value, that shouldn't be used. 

**To Reproduce**

``` python
import montepy
mat= montepy.Material()
mats = montepy.materials.Materials()
mats.append_renumber(mat)
print(mat.number)
```

yields

```
-1
```


**Version**

 - Version 0.5.3, 1.0.0a1
