Made the nodepaths relative by default

The textnodes in the output-xdmf-file now show the relative path of the node they belong to. We added a command line switch, which allows to print the full nodepath to the textnodes by adding "--fullpath" as parameter when you run splash2xdmf.py.

closes [106](https://github.com/ComputationalRadiationPhysics/libSplash/pull/106)