Added magnitudes as init attribute for CartesianGrid2D class

[pabloitu]

The magnitude attribute it is now an argument of the CartesianRegion2D class, and not left to the user to hardcode it after class instantiation. It is optional (defaults to None), in case the region is used exclusively in the spatial sense.

fix: Added magnitudes as init attribute for CartesianGrid2D class. Defaults to none. Changed default CSEP regions instantiation to account for this, instead of hardcoding magnitudes.

tests: added magnitude instantiation in class unit tests.

closes #257

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Fixes issue #(please fill in or delete if not needed).

Type of change:

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• Bug fix (non-breaking change which fixes an issue)

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• I have performed a self-review of my own code
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• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[Copilot]

Pull Request Overview

This PR adds a new optional "magnitudes" attribute to the CartesianGrid2D class and updates related instantiations across the code to use this parameter instead of assigning it after initialization. Key changes include:

• Updating the CartesianGrid2D constructor to accept an optional magnitudes parameter.
• Modifying region-builder functions in csep/core/regions.py to pass magnitudes directly.
• Adjusting unit tests in tests/test_spatial.py to include a magnitudes array during grid instantiation.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
tests/test_spatial.py	Updated grid instantiation to include magnitudes in unit tests
csep/core/regions.py	Refactored region creation functions to pass magnitudes