refactor: replace `&mut self` by `mut self` in `Integraal` setters signatures

[imrn99]

also, change &mut Self to Self as the return type for these setters

closes #19

Content

• Scope:
  • [x] Code: if relevant, add affected target
• Type of change:
  • [x] Refactor
• Other:
  • [x] Breaking change

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 92.18%. Comparing base (dd304f7) to head (8de2017).

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## trunk #22 +/- ## ========================================== + Coverage 92.15% 92.18% +0.02% ========================================== Files 3 3 Lines 306 307 +1 ========================================== + Hits 282 283 +1 Misses 24 24 ```

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