Improve coverage, setup to ignore flex

[michael-petersen]

This PR does some test improvement, and also adds an example of how to ignore a specific directory with pytest.

[Copilot]

Pull request overview

This PR improves test coverage for the discmodel package and adds pytest configuration to ignore the flex directory during test runs. The changes include expanded test cases for the expansion functions, a bug fix in the optional imports checker, and modifications to exception handling in the disc rotation code.

• Adds test coverage for noisy expansions and expansion computation before image generation
• Fixes bug in _check_flex() function that was checking for wrong module
• Comments out try-except blocks in rotation code that handled single-particle edge cases
• Configures pytest to ignore the flex directory

Reviewed changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
tests/test_discmodel.py	Adds test for expansion before image generation and noisy expansion test; renames variable E2 to E3 for clarity
src/discmodel/optional_imports.py	Fixes bug where _check_flex() was incorrectly checking for "lintsampler" instead of "flex"
src/discmodel/discmodel.py	Comments out try-except blocks in rotate_disc() that handled single-particle (N=1) edge cases
pytest.ini	Adds pytest configuration to ignore the flex directory during test runs
.github/workflows/test.yaml	Minor whitespace cleanup in pytest command

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