add a new tool which implements Rosin's algorithm to compute threshold of an unimodal intensity histogram

[vanthonguyen]

Thanks a lot for contributing to DGtalTools-contrib, before submitting your PR, please fill up the description and make sure that all checkboxes are checked.

PR Description

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Checklist

• [ ] Doxygen documentation of the code completed (classes, methods, types, members...)
• [ ] Check if it follows the tools structure described in CONTRIBUTING.md
• [ ] New entry in the ChangeLog.md added.
• [ ] Update the readme with potentially a screenshot of the tools if it applies.
• [ ] No warning raised in Debug cmake mode (otherwise, Travis C.I. will fail).

[kerautret]

I think we can merge, just add the new tools in readme but I will put it afterwards.