GEOPY-2866: Research auto-scaling strategy for cross-gradient regularization terms

[domfournier]

GEOPY-2866 - Research auto-scaling strategy for cross-gradient regularization terms

[Copilot]

Pull request overview

This PR updates the cross-gradient regularization to use the standard SimPEG weight infrastructure (via set_weights / W) and introduces a new joint-inversion directive intended to scale cross-gradient weights based on model-gradient derivative magnitudes.

Changes:

• Refactors CrossGradient to construct and apply its weighting operator through a new W property and stored weights (including cell volumes).
• Adds a new inversion directive to rescale cross-gradient weights at the end of each iteration.
• Exposes the new directive through simpeg.directives.

Reviewed changes

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

File	Description
simpeg/regularization/cross_gradient.py	Moves cross-gradient weighting to set_weights + cached W operator; updates default units value.
simpeg/directives/_sim_directives.py	Adds a directive that computes a scale from model ranges and updates CrossGradient weights each iteration.
simpeg/directives/__init__.py	Re-exports the new directive from the top-level directives package.

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