fix: gotcha with thin disc lineprofiles

[codecov-commenter]

Codecov Report

Patch coverage: 76.92% and project coverage change: +0.02% :tada:

Comparison is base (5715049) 68.46% compared to head (004b826) 68.49%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #177 +/- ## ========================================== + Coverage 68.46% 68.49% +0.02% ========================================== Files 66 66 Lines 2775 2787 +12 ========================================== + Hits 1900 1909 +9 - Misses 875 878 +3 ``` | [Files Changed](https://app.codecov.io/gh/astro-group-bristol/Gradus.jl/pull/177?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=astro-group-bristol) | Coverage Δ | | |---|---|---| | [src/line-profiles.jl](https://app.codecov.io/gh/astro-group-bristol/Gradus.jl/pull/177?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=astro-group-bristol#diff-c3JjL2xpbmUtcHJvZmlsZXMuamw=) | `72.41% <71.42%> (-0.32%)` | :arrow_down: | | [...ransfer-functions/cunningham-transfer-functions.jl](https://app.codecov.io/gh/astro-group-bristol/Gradus.jl/pull/177?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=astro-group-bristol#diff-c3JjL3RyYW5zZmVyLWZ1bmN0aW9ucy9jdW5uaW5naGFtLXRyYW5zZmVyLWZ1bmN0aW9ucy5qbA==) | `96.25% <83.33%> (-0.53%)` | :arrow_down: |

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