Non-zero thickness implementation

[michael-petersen]

This pull request enhances the usability and documentation of the discmodel package, focusing on clearer code, improved docstrings, and better support for modeling disc galaxies with vertical structure. The most significant changes are the addition of isothermal slab thickness support, improved documentation and usage examples, and expanded test coverage.

Support for vertical disc thickness:

• The DiscGalaxy class now supports an isothermal vertical slab via the new zscale parameter, allowing users to model discs with finite thickness. By default, a small nonzero thickness is used unless zscale=0.0 for a flat disc. The sampling of positions (z) and velocities (w) now reflects this vertical structure.

Improved documentation and code clarity:

• Comprehensive docstrings were added to the main class and methods (DiscGalaxy, make_rotation_matrix, rotate_disc, generate_image, make_expansion, make_particle_expansion, resample_expansion, compute_a1), clarifying input parameters, behavior, and usage.

Testing enhancements:

• New tests were added to verify that the isothermal slab thickness is correctly sampled and that setting zscale=0.0 produces a flat disc as expected.

[Copilot]

Pull request overview

This pull request extends discmodel.DiscGalaxy to support sampling a finite vertical thickness (Gaussian/isothermal-slab-style), while also improving docstrings and expanding user-facing documentation and tests.

Changes:

• Added vertical thickness sampling via a new zscale parameter, including corresponding vertical velocity dispersion sampling.
• Expanded/clarified docstrings across key DiscGalaxy methods.
• Added tests and README usage examples covering nonzero and zero-thickness discs.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/discmodel/discmodel.py	Implements vertical thickness sampling (z, w) and adds/updates method docstrings.
tests/test_discmodel.py	Adds tests for nonzero zscale sampling and for zscale=0.0 flat-disc behavior.
README.md	Improves code fences and adds an example for visualizing generated images plus zscale usage notes.

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