Added harmonic filter

[TheLittleChosenOne]

Description

Checklist

• A descriptive but concise pull request title
• Docstrings for all functions
• Unit tests
• A gallery example in ./doc/examples for new features
• Contribution guide is followed

Release note

Summarize the introduced changes in the code block below in one or a few sentences. The
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...

[mkcor]

Dear @TheLittleChosenOne,

Thank your for your contribution. It looks like you are implementing a new feature; is there a related feature request in the open issues? It would be great if you could share your use case for this filter.

If we choose to support this new feature, we would need to add tests for the function and create a gallery example (based on your use case?). Thanks again.

[TheLittleChosenOne]

Hello mkcor,

I will be honest and tell you that I found this project and found it very interesting and made a contribution as one of my HW assignments. I am not sure if I am allowed to share the use case for this filter. By all means if this contribution is going to help then you can add it and make which ever test cases you can if it will help the project.

Thanks

[stefanv]

Thanks for the contribution @TheLittleChosenOne.

This is presumably the Harmonic mean filter. You can see an example of its application (from the Gonzales & Woods textbook) in these slides.

I am not sure these filters are used much in practice? We could consider adding it if there's a demonstration of need—they are fairly straightforward to implement. The implementation here would need to be changed, though, since it assumes a fixed mask size and iterates over elements in a double for-loop.