Fix Gaussian kernel issue

[sfarrens]

closes #272

• Following this Astropy issue, it is no longer possible to return an unnormalised Gaussian kernel. Therefore, I have removed this option from gaussian_kernel and the corresponding test.
• Following this Scikit-Image issue, a maximum version of the package has been temporarily been set for the tests.

[codecov-commenter]

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[paquiteau]

Thanks for the PR,

only a docstring update is missing, otherwise, I am good with it.

[sfarrens]

Hey @paquiteau, if you are happy with this can you click on the "Approve" button? 🙂