atmchange/atmquery use case for missing initial condition

@AaronDonahue wanted to do an atmchange on `initial_conditions::qi`.

```
% ./atmchange initial_conditions::qi=0.0
ERROR: initial_conditions::qi did not match any items
```

The reason is `initial_conditions::qi` is not in namelist_scream.xml is because in namelist_defaults_scream.xml:

```
0.0
```

The `grid` check fails so `qi` is just omitted and then is not usable by atmchange. We maybe need a smarter system to handle this case (qi is in the xml but is flagged as skip?) or the ability to append child elements, probably the latter.