Automatically delete files marked as temp as soon as not needed anymore

To reduce the footprint of larger workflows it's very useful if temporary files (which are marked as such) could be automatically deleted once they are not used anymore. Yes, this breaks reruns, but for easily recomputed files or large ones (footprint), this makes sense. Using scratch (see #230) is not always possible/wanted (e.g. for very large files and small scratch). It's also not always possible to delete those files as a user (except at the very end of the workflow), because multiple downstream processes running at different times might require them. This feature is for example implemented in snakemake, but maybe it's easily done there because the DAG is computed in advance?

Note, this is different from issue #165 where the goal was to remove non-declared files. The issue contains a useful discussion of the topic nevertheless.

Andreas