Add requirements.txt file for creating virtual env for running Prefactor

Added a requirements.txt file to create a virtual environment, which contains toil-cwl-runner and cwltool as CWL runners.

requested review from @mancini

assigned to @loose

enabled an automatic merge when the pipeline for 107ee97f succeeds

mentioned in commit 3381dbfb

merged

mentioned in commit 227847a7

mentioned in commit 70d9bb66

mentioned in commit d090b186

mentioned in commit 2916628b