fix broken links

[ChrisJefferson]

fix broken links

[felixvuo]

I would like to contribute to csplib if possible, so I've done a broken link check, forked the repository and begun to update links where I can find more up-to-date info. I'll put together a pull request when I feel I've done what I can (which may not be much).
Broken Links courtesy of brokenlinkcheck.com.pdf

[ozgurakgun]

Thanks @felixvuo that would be very helpful.

[arnaud-m]

Hi @ozgurakgun,

I was looking for a way to contribute again to the csplib and this issue looks like a good starting point.

So, I have executed the W3C link checker.

1. Many internal links miss trailing slashes, and caused a redirect. Adding the trailing slash would speed up browsing.
2. Some other links are also redirect from http to https.
3. Some buttons 'edit page' (see here) are broken.
4. Last, I have stopped the link checker after it has found more than sixty broken external links.