Fix isort import-ordering in curvature.py

[brendancol]

Closes #3388

isort --check-only xrspatial/curvature.py failed: the module had nine
separate from xrspatial.utils import ... lines with
xrspatial.dataset_support ordered after them.

• Collapse the xrspatial.utils imports into one parenthesized,
  alphabetically sorted import.
• Order xrspatial.dataset_support before xrspatial.utils, keeping the
  # local modules comment above the first local import.
• No behavioural change — import-ordering only.

Backends: not applicable (no code path touched).

Test plan:

• isort --check-only xrspatial/curvature.py clean
• flake8 xrspatial/curvature.py clean
• pytest xrspatial/tests/test_curvature.py (80 passed)

[brendancol]

PR Review: Fix isort import-ordering in curvature.py

Blockers (must fix before merge)

None.

Suggestions (should fix, not blocking)

None.

Nits (optional improvements)

None.

What looks good

• The diff is import-ordering only; no executable code path is touched.
• The set of imported names is unchanged from main (10 names in, 10 out): dataset_support.supports_dataset plus the nine xrspatial.utils names. flake8 reports no unused or missing imports, so nothing was dropped or orphaned in the collapse.
• # local modules is kept above the first local import, matching the request in #3388 rather than isort's default of burying the comment between the two import lines.
• isort --check-only and flake8 are both clean; existing test_curvature.py (80 tests) passes.

Checklist

• No behavioural change (import-ordering only)
• Imported name set identical before/after
• isort clean
• flake8 clean
• Existing curvature tests pass
• New tests — not applicable (no behaviour change)
• README / docs — not applicable (no API change)
• Benchmark — not applicable