Support for transient data

### User Story Description

As a user I want to be able to load transient datasets and control the time step displayed.

Currently unsupported in AVZ (unofficially).

Note that currently we have the request only for one time set. So the animation control should work only on one timeset. It is in theory possible that in the future we will have requests to support multiple timesets in the same model (think about animated streamlines on a transient model), but it currently isn't needed

### Acceptance Criteria

Launch a session of VISOR. Load a transient dataset. The VISOR session shows a time display with the option to move between timesteps and play the entire time sequence.

Go to a specific timestep. Save a state file. In a new session of VISOR, load that state file. You are at the correct timestep.

### Additional Links

_No response_

### Requires new or update to third party ? If "yes" add the name(s) and the version(s) of the Third Party Software.

No

### Name and Version for Third Parties

_No response_

Start by tracing VISOR session startup, transient dataset loading, animation controls, and state-file save/load behavior. Verify the result by loading one transient timeset, moving and playing through its timesteps, then reopening a saved state at the same timestep.

Written by the indexing model from the issue text.