Warning in derived fields example

[kyleaoman]

I get the following warning when running the examples in the 'derived fields' section of the docs:

>>> ds['stars']['VelMag']
/home/txwx36/.venv/scida/lib64/python3.11/site-packages/dask/array/core.py:3469: UserWarning: Passing an object to dask.array.from_array which is already a Dask collection. This can lead to unexpected behavior.

[cbyrohl]

This issue is related to pint. For xarray they discussed the same issue of pint interplay with dask: xarray-contrib/pint-xarray#116 and pydata/xarray#5559. Short-term, I will add this warning to the FAQ and/or suppress error. Long-term, need to check how xarray fixed this in this PR: pydata/xarray#5571

[cbyrohl]

I adjusted the example to not produce this warning in #122