[demianw]

What was wrong

Some tract_math operations import packages that are not declared anywhere: SciPy (scipy.ndimage in tract_map_image, tract_deform, tract_generate_population_probability_map; scipy.interpolate in tract_prototype_mean) and scikit-learn (sklearn.neighbors.BallTree in tract_smooth). Without them, users get a bare ModuleNotFoundError: No module named 'scipy' at run time.

The change

• pyproject.toml: new optional extra tract_math = ["scipy", "scikit-learn"] (4 added lines, nothing else changed).
• operations.py: a small _import_optional(module_name) helper (not a registered operation) used by the four operations above; when the module is missing it raises ImportError: This operation needs scipy: install it, or install tract_querier with the tract_math extra, e.g. pip install '.[tract_math]' from the source tree (chained to the original error). tract_prototype_mean keeps its existing behaviour (warns and skips the smoothing without SciPy); only its warning now names the extra.
• doc/installation_instructions.rst: one sentence after the requirements list.

The message suggests pip install '.[tract_math]' rather than tract_querier[tract_math] because the package is not on PyPI (the docs install from GitHub or a source tree).

tensor_operations.tract_expand_tensor_metrics also imports SciPy, but it is dead code (undefined name, no callers) and is removed in the #107 PR, so it is left alone here. tract_label_indices.py imports SciPy too, outside tract_math (handled by #135).

Testing

• New parametrized test_tract_math_missing_optional_dependency: hides scipy/sklearn via sys.modules and checks that tract_map_image, tract_deform and tract_smooth raise an ImportError naming the tract_math extra. Passes here.
• python -m pytest -q -p no:cacheprovider tract_querier: 45 passed, 1 skipped (pre-existing unconditional skip).
• I did not build the docs for this one-sentence change.

Maintainer decision: CI installs .[test] only, so SciPy-dependent tests (e.g. those in #143, which use pytest.importorskip) are skipped there; installing .[test,tract_math] in .github/workflows/test_package.yaml would run them. Not changed here.

Merge order

pyproject.toml is also edited by #98 (moves VTK to an optional extra in the same [project.optional-dependencies] table, so it will conflict textually next to this one), #120 and #121 (other tables; probably merge cleanly). doc/installation_instructions.rst is also edited by #98 (requirements list, adjacent lines). operations.py is also edited by #143 and #145 (different lines).

Closes #112

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