Get stresses from simulation

### Description of the feature

Get stresses (nodal, elemental and raw) from simulation data.

Return a DataObject with the requested info.

The users should be able to directly request:

```py
nodal_stress = simulation.nodal_stress()
elemental_stress = simulation.elemental_stress()
raw_stress = simulation.raw_stress()
```

And to select by nodes, elements, named_selections, etc.

### Steps for implementing the feature

_No response_

### Useful links and references

N/A

Start from the simulation object and its existing result-access APIs; the issue does not identify specific files or tests. Define the nodal_stress(), elemental_stress(), and raw_stress() entry points, ensuring each returns a DataObject and supports selection by nodes, elements, and named selections.

Written by the indexing model from the issue text.