Improve Test Coverage for levels.py

[harryswift01]

Summary

This PR aims to improve the test coverage of the levels.py module by implementing atomic tests for its classes and functions. The goal is to ensure the correctness of logic and numerical computations throughout the module.

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Changes

Restructured Testing Order

• Adopted a class-based structure for test cases, aligning with the format used in other parts of the codebase.
• Improved readability and maintainability by grouping related tests.

Expanded Unit Tests for Core Functions

• Added atomic tests for all major functions and classes in levels.py.
• Included tests for the retrival of MDAnalysis Universes as well as geometric calculations.
• In places where it is mathematically improbable to achieve, and difficult to test cases for # pragma: no cover has been utilised to signal this is the case

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Impact

• Significantly increases test coverage for levels.py, improving confidence in the module’s reliability.
• Helps catch regressions and logic errors early in development.
• Aligns testing practices with the rest of the codebase for consistency.

[jimboid]

Great PR, I see you solve the pragmas! Tests look comprehensive