Add document to TAD and IS track

[Nanguage]

Please add document string to new class:

https://github.com/GangCaoLab/CoolBox/blob/7bceb42025b4ef1b32a6bcc84dea0c2f64135610/coolbox/core/track/hist/hicfeatures.py#L145

And an usage example like:

https://github.com/GangCaoLab/CoolBox/blob/master/docs/source/_gallery/v4c.ipynb

Then include it in this file:

https://github.com/GangCaoLab/CoolBox/blob/master/docs/source/gallery.rst

and re-generate the html document:

coolbox/docs/ $ make html; cp -r build/html/* .

@zhqu1148980644

[Nanguage]

We fellow this doc-string style: https://numpydoc.readthedocs.io/en/latest/format.html