Add support in convert.nemo_to_sgrid for depthw coordinate

[VeckoTheGecko]

Description

Related to getting the benchmark suite running (including the prior PR - #2605 ) this PR does the following:

• Adds a test test_convert.py::test_nemo_to_sgrid_with_depth
• Adds a metadata-only version of the original source dataset in the benchmark repo (for use in test_nemo_to_sgrid_with_depth)
• Updates the convert module to allow for the depthw dimension to be optionally provided
• Adds type hinting to the convert module allowing for easier refactoring/overview

Still need to double check that this fixes all the problems in the benchmark repo - will do that on Wednesday. If you could review the code in the meantime that would be great @erikvansebille :)

Checklist

• Closes None
• Tests added
• This PR targets the correct branch (main for normal development, v3-support for v3 support)

AI Disclosure

None used

[VeckoTheGecko]

Note

Refactor the whole convert module, this can surely all be handled better.

Is still on my TODO list (though quite low down). I have ideas on how we can make the conversion very robust but I want that to come later.

[VeckoTheGecko]

Still need to double check that this fixes all the problems in the benchmark repo - will do that on Wednesday

I just did, and I think its working - though didn't have the time to wait for the run to complete. I think that's the last of the bughunting - working on perf on Wed