Fix/doc graphviz inheritance in pdf

OK, so I found the place to declare the dpi resolution of graphviz class inheritance graphs. I've set it to dpi=300. Also the font was very pixelated, so I also had to change the fontsize. It is all in lines 85 and 86. The generated Yade.pdf file is only 45kb bigger, so it's not a big deal. And graphs are now useful and readable :)

It is also an occasion to maybe increase the fontsize in inheritance graphs in the html. I think it is a little bit too small. But I didn't change it.

When I make this merge request, I am not sure why it keeps dragging with it all the earlier commits, which do not belong here. This merge request is only the last commit. Ahh, I relized now - I should have forked directly from master, not from fix/examples. I hope this will not cause rebase problems. So I learned now another thing about making merge requests on gitlab :)