Allow `==` if one of the operand is a thin interval

[OlivierHnt]

Closes #628.

[codecov-commenter]

Codecov Report

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

Project coverage is 82.99%. Comparing base (5275e5c) to head (83d8c1e).

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #631 +/- ## ========================================== + Coverage 82.92% 82.99% +0.06% ========================================== Files 26 26 Lines 2173 2176 +3 ========================================== + Hits 1802 1806 +4 + Misses 371 370 -1 ```

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