`mesh_utils` methods are not returning necessary information.

### 📝 Description of the feature

While testing in PR #483 I've seen that most methods do not return any value. After reviewing this with @RobPasMue , these methods should be returning information on what the function has modified in the `Model` class. Not having this makes verifying that the method performed what you wanted to do impossible at user level.

For example, `Mesh.merge_parts` could return the final list of parts of the model.

### 💡 Steps for implementing the feature

Add returns with what has been modified in the `Model` to the `Mesh` class.

### 🔗 Useful links and references

https://github.com/ansys/pyprimemesh/blob/main/src/ansys/meshing/prime/lucid/mesh_util.py

Start with src/ansys/meshing/prime/lucid/mesh_util.py and review the Mesh methods discussed in PR #483, especially Mesh.merge_parts. Identify what each method modifies in the Model class and define the corresponding return information; done means the relevant methods return that information so callers can verify the changes at user level.

Written by the indexing model from the issue text.