add a normalization test

[michael-petersen]

This pull request makes some improvements to the flex package, focusing on input validation, array handling, and code structure. The changes enhance robustness, improve maintainability, and modernize the package setup.

Refactoring and Modernization

• Removed the legacy setup.py file in favor of a modern pyproject.toml-based build system.
• Updated pyproject.toml to specify the package source directory as src, added configuration for package discovery, and updated the project homepage URL.

Performance and Code Simplification

• Refactored the computation of trigonometric arrays and summations in laguerre_amplitudes, laguerre_amplitudes_newaxis, and laguerre_reconstruction to use efficient broadcasting and numpy operations instead of Python loops, improving performance and code clarity.

Testing Improvements

• Replaced the old normalization test with a more comprehensive test that checks the accuracy of the Laguerre reconstruction against a known peak amplitude, increasing test coverage and reliability.

[Copilot]

Pull request overview

This PR updates the flex Python package by tightening input validation in the core FLEX class, modernizing packaging to a pyproject.toml-based build, and replacing an older normalization-oriented test with a reconstruction accuracy test to better exercise the numerical path.

Changes:

• Added centralized numeric input coercion/validation for FLEX inputs (R, phi, mass, velocity, plus reconstruction inputs).
• Refactored Laguerre amplitude and reconstruction computations to use NumPy broadcasting/einsum instead of explicit Python loops.
• Modernized packaging metadata (remove setup.py, update pyproject.toml config) and adjusted test dependencies accordingly.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tests/test_flex.py	Replaces the previous normalization-focused test with a reconstruction peak-amplitude check to validate the updated reconstruction path.
test_requirements.txt	Removes numba from test requirements and keeps editable install (-e .) for the src-layout package.
src/flex/flexbase.py	Adds input validation helpers and rewrites amplitude/reconstruction logic with vectorized NumPy operations.
setup.py	Removed in favor of pyproject.toml build configuration.
pyproject.toml	Removes unused build/runtime dependencies and configures setuptools package discovery for the src/ layout (plus pytest config).

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