Fixes #734: make noise a lazy import in datasets module

[brendancol]

Summary

• Move import noise from top-level in xrspatial/datasets/__init__.py to inside make_terrain(), guarded by try/except
• Previously, from xrspatial.datasets import get_data would fail with ModuleNotFoundError: No module named 'noise' on a fresh pip install xarray-spatial since noise is only in [options.extras_require] tests
• Now the module imports cleanly without noise, and make_terrain() raises a clear ImportError with install instructions

Test plan

• Verified xrspatial.datasets imports successfully without noise installed
• Verified make_terrain() works when noise is installed
• Verified make_terrain() gives a clear error when noise is missing: ImportError: make_terrain requires the 'noise' package: pip install noise