Enable easier copying into MutableMapping

From https://github.com/ansys-internal/pyacp/pull/258#discussion_r1295546421

Make it easier to copy children from one object to another. Currently, the `MutableMapping` doesn't support
`__setitem__`, which means e.g. the following doesn't work:

```python
# mg1 and mg2 are Modeling Groups
for ply_id, ply in mg1.modeling_plies.items():
mg2.modeling_plies[ply_id] = ply.clone()
```

We can either implement `__setitem__` [1], or e.g. add a dedicated `clone_from` method.

Consider also whether adding a parameter to the parent object's constructor makes sense (e.g., `modeling_plies` in the `ModelingGroup.__init__`). One maybe non-obvious behavior of this is that the children would then (unless they were previously unstored) have to be _new_ objects, not the passed ones.

[1] the complication being how to ensure (and make it easy-to-use) that the key matches the new ID.

Start by reading the MutableMapping implementation and the ModelingGroup.modeling_plies usage shown in the issue, then review the discussion in pull request 258. Decide whether __setitem__, clone_from, or constructor support best defines the key and new-ID behavior. Done means the reported copy loop has a clear, consistent API and the chosen behavior is documented.

Written by the indexing model from the issue text.