Add discrete differencing operators for gradient, curl and divergence methods.

### Is your feature request related to a problem?

Optimising existing `NEMODataTree` methods.

### Describe the solution you'd like...

- Utilise `.step()` and `.roll()` for grid differencing.
- Updating `coords` to resulting model grid (i.e., `i`, `j`, `k`).

### Describe any alternatives you've considered...

N/A

### Additional context

_No response_