Scale gradient and curl by sphere radius

[rajeeja]

Summary

• scale gradient() and curl() by uxgrid.sphere_radius by default
• add scale_by_radius=False for unit-sphere derivatives

Closes #1516

[Copilot]

Pull request overview

This PR updates UXarray’s vector calculus API so gradient() and curl() return physically-scaled derivatives by default (using uxgrid.sphere_radius), while still allowing unit-sphere derivatives via scale_by_radius=False.

Changes:

• Add scale_by_radius parameter (default True) to UxDataArray.gradient() and UxDataArray.curl().
• Apply 1/radius scaling inside the shared _compute_gradient() implementation.
• Update and add tests to validate both unit-sphere and radius-scaled behavior.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 5 comments.

File	Description
uxarray/core/gradient.py	Adds optional scaling by uxgrid.sphere_radius to gradient computation.
uxarray/core/dataarray.py	Exposes scale_by_radius in public gradient()/curl() APIs and forwards it into _compute_gradient().
test/core/test_vector_calculus.py	Updates existing gradient tests to use unit-sphere behavior and adds new scaling-by-radius assertions for gradient/curl.

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