Add tutorial API for example datasets

[alioacar]

Closes #1496

This is a draft PR for early feedback on the tutorial dataset API. I also went through the notebooks and updated them to use tutorial API.

Overview

• Add a new ux.tutorial API for loading built-in sample datasets from the existing test/meshfiles directory (following @erogluorhan's suggestion)
• Add a tutorial dataset registry under uxarray/tutorial/
• Add tests covering the new tutorial helpers
• Update the notebooks to use the tutorial API
• In data-structures.ipynb, show both the usual dataset(and grid) loading pattern and the tutorial API for comparison

Expected Usage

import uxarray as ux

# tutorial dataset usage
uxds = ux.tutorial.open_dataset("quad-hexagon")
uxgrid = ux.tutorial.open_grid("outCSne30")

# tutorial dataset usage with multiple data files
uxds_multi = ux.tutorial.open_mfdataset("quad-hexagon-random")

# regular usage with user-provided files remains unchanged
grid_path = "/path/to/grid.nc"
data_path = "/path/to/data.nc"
data_paths = ["/path/to/data1.nc", "/path/to/data2.nc"]

uxds = ux.open_dataset(grid_path, data_path)
uxds_multi = ux.open_mfdataset(grid_path, data_paths)

PR Checklist

General

• An issue is linked created and linked
• Filled out Overview and Expected Usage (if applicable) sections

Testing

• Adequate tests are created if there is new functionality
• Tests cover all possible logical paths in your function
• Tests are not too basic (such as simply calling a function and nothing else)

Documentation

• Docstrings have been added to all new functions
• Update api.rst

Examples

• Clear the output of all cells before committing

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[erogluorhan]

Thanks @alioacar ! @rajeeja and I will look into this as soon as we can and will get back to you. In the meantime, I approved the workflow runs, we'll see their results soon.

[rajeeja]

Thanks for this PR, @alioacar _meshfiles_path() resolves to test/meshfiles/ which is not included in the PyPI distribution, so ux.tutorial.open_dataset() will raise FileNotFoundError for any user who did pip install uxarray rather than cloning the repo, please revise uxarray/tutorial/init.py, in the _meshfiles_path() function.. Path.cwd() is not the rightway.

[alioacar]

thanks for the feedback @rajeeja. I updated the tutorial data path resolution.
1- use an env variable for data path. UXARRAY_DATA_DIR (for docs or other configured envs)
2- otherwise use the local test/meshfiles directory (working from a clone)
3- otherwise fall back to download/cache for PyPI.
This removes the reliance on Path.cwd while still keeping the cloned workflow working.

[rajeeja]

Thanks for the contribution👍✈️