do not use dask if no scheduler is given and `n_jobs=1`

[kain88-de]

Currently we use dask even to a serial job. This can make debugging quite hard since it breaks the python debugger expcetations. If we setup that serial jobs run without dask it will be easier to people to develop new algorithms and debug them with the python debugger.

[orbeckst]

We should also allow the use of the single-threaded scheduler for debugging.