[Code scan] Clean up non-portable example notebooks and unpublished pip command

[njzjz]

This issue comes from a Codex global repository scan.

Problem

Some examples are not reproducible from a fresh checkout.

example/dftio_demo.ipynb executes Field.from_cube("xxx.cube"), but no xxx.cube fixture exists in the repository:

dftio/example/dftio_demo.ipynb

Line 87 in c9d128f

"f = Field.from_cube(\"xxx.cube\")"

The SIESTA notebooks contain private absolute paths and stale imports, so they fail outside the author's machine:

dftio/dftio/io/siesta/try_siesta.ipynb

Line 11 in c9d128f

"struct = sisl.get_sile(\"/personal/DeepTB/dptb_Zjj/DFTIO/dftio/test/data/siesta/siesta_out/RUN.fdf\").read_geometry()\n",

dftio/dftio/io/siesta/try_siesta.ipynb

Line 65 in c9d128f

"from ...data import _keys\n",

dftio/dftio/io/siesta/check_siesta.ipynb

Lines 9 to 12 in c9d128f

	"from dptb.data import build_dataset\n",
	"\n",
	"dataset = build_dataset(\n",
	" root=\"/personal/DeepTB/dptb_Zjj/DFTIO/dftio/test/data/test_siesta2\",\n",

The README also shows an uncommented pip install dftio command under a “coming soon” heading, which fails while the package is not published to PyPI:

dftio/README.md

Lines 42 to 46 in c9d128f

	### Using pip (from PyPI - coming soon)
	```bash
	pip install dftio
	```

Suggested fix

Add a small cube fixture or make that notebook cell non-executed, replace private paths/stale imports in SIESTA notebooks with repo-relative examples, and remove or comment out the PyPI install command until publication.