Changed make coverage so as not to cause errors.

Proposed changes

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Types of changes

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[ ] Bugfix (non-breaking change that fixes an issue)
[ ] New feature (non-breaking change that adds functionality)
[ ] New module (non-breaking change that adds a new module)
[ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
[ ] Documentation update

Checklist

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[ ] I have added tests that prove my fix is effective or that my feature works
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[ ] (Optional) I have added an example showcasing the new change

Further comments

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VirtualPlanetaryLaboratory / vplanet