Fixed link building

[patrick-sil]

This PR is supposed to resolve #123

However, let me add to the description of the issue above that it also applies to internal references inside the PDF, hence, currently :ref: or :numref: are not working for sub-headlines in any case.

The suggested changes here, should fix the issue for both the HTML and the PDF.

What does my solution do:

1. Introduces a new function which
2. Does check on whether the link actually contains a #
3. If so, it does only take the part of the link of the right-hand side of the last, i.e., right most #
4. Return the now fixed link or the original link (also works for cases where only one # is present in the link, as it basically doen't modify the link in such cases)

If anyone has a better solution, which I assume are possible, as I'm not all that familiar with this project, please suggest them.

It would be nice, if this could be resolved soon, thank you :)