test: add main structure coverage

[imrn99]

Write new tests in the dedicated submodule; They cover both correct and incorrect usages of the struct, though incorrect usages are most likely not exhaustive.

Also add some trait implementations that are required for assertions and debugging.

Content

• Scope:
  • [x] Code
• Type of change:
  • [x] Testing
• Necessary follow-up:
  • [x] Testing: the second integral used to test is incorrectly computed because of grey areas in how discretization / rectangles / trapezoids are handled

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 93.39623% with 7 lines in your changes are missing coverage. Please review.

Project coverage is 60.75%. Comparing base (5272b52) to head (49e2dcf).

Files	Patch %	Lines
src/structure/tests.rs	93.39%	7 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## trunk #6 +/- ## ========================================== + Coverage 3.75% 60.75% +57.00% ========================================== Files 9 9 Lines 240 344 +104 ========================================== + Hits 9 209 +200 + Misses 231 135 -96 ```

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