fixing doc for Hull2D namespace

[troussil]

Thanks a lot for contributing to DGtal, before submitting your PR, please fill up the description and make sure that all checkboxes are checked. Please remove these lines in your PR.

PR Description

The documentation has been fixed for Hull2D namespace, which appears now only in namespaces DGtal::functions with a brief description.

Checklist

• Unit-test of your feature with Catch.
• [ x] Doxygen documentation of the code completed (classes, methods, types, members...)
• Documentation module page added or updated.
• New entry in the ChangeLog.md added.
• No warning raised in Debug cmake mode (otherwise, Travis C.I. will fail).
• All continuous integration tests pass (Travis & appveyor)

Other items are not relevant

[kerautret]

@troussil appveyor is failing but PR #1326 should fix it ;)

[dcoeurjo]

thanks, merging.