Fix dask setup to *not* prescribe local directory

The dask setup for local cluster at the moment prescribes a fixed location on a ram disk. While the use of a ram disk might be beneficial and certainly not all (HPC) file systems are equally suited for the task, the prescription of one particular directory is to restrictive and creates problems on shared environments. For now, let's revert to dask choosing the directory, possibly giving a configuration option later on.