And updating tutorial

ANd expading comments on loading in the dataset

To use same ordering as in XLinear

This requires smaller selection_dict for the isel, so hopefully faster code

And also separating offset calculation into its own helper function

It's not clear why this is here, nor why removing it causes a test failure. To be investigated another time.

EDIT: This was introduced in
Parcels-code@c311fba
- though we're investigating if this can be implemented another way
  since there should be no particular difference with NEMO

Gradually reducing the dependency on the `mesh` param

Updates the API for conversion to be more closely aligned with the input data. Also handles the U and V fields separately - correctly assigning the dimension naming before merging into a single dataset.

Update tests

Avoid assuming there's a U and V field. Maybe this should be refactored later...

Avoiding a FutureWarning: The return type of `Dataset.dims` will be changed to return a set of dimension names in future, in order to be more consistent with `DataArray.dims`. To access a mapping from dimension names to lengths, please use `Dataset.sizes`.

…o-nick

Remove **fields and replace with normal arg