Fix bug#1813229. Class Inheritance graphs in Yade.pdf now have dpi=300 and larger font.

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.

I made a second merge request, this time only with this single commit. To make it more clear.