Don't write to parent dir when rendering problem statement

[gkreitz]

I got bit by this just now, so I decided to fix it. Our archaic setup to render pdf:s prior to this PR involves writing some temporary files (problemset.cls and a .tex file with a random name) to the parent directory of the problem we're installing). This obviously fails badly if the user running problemtools does not have write access above the problem directory.

This PR puts the temporary files in a temporary directory instead. To be able to do so, I had to add a new feature to the cls file, with the new command \problemparentpath{<path>}, which makes the other commands look for problems in that path, rather than in the current directory.