Switch dune-pdelab-tutorials to using UseLatexMk.cmake

Over coffee breaks, we love cursing build systems. Building PDFs from cmake has been a particularly popular topic.

Steffen and me have come up with an alternative way of doing this. Instead of using UseLATEX.cmake as shipped by dune-common (originally from https://github.com/kmorel/UseLATEX), we build a solution on latexmk. We are convinced that the result will be superior. I will publish it on github during the next days.

As with previous innovations, we would like to use PDELab as a testing playground for the core modules.

Have a look at how those PDFs are built exactly how they are supposed to be without maintaining a list of symlinks to C++ sources!