Availability of cwltool

[AsierGonzalez]

The VRE_RUNNER bash script does two things that seem contradictory:

1. Checks if cwltool is available: type -a "$prog" >/dev/null in line 17
2. Activates virtual environment: source "$BASEDIR"/venv/bin/activate in line 26

These two steps are contradictory because for cwltool to be available it has to be installed globally or the virtual environment has to be activated first. I don't think we want cwltool to be installed globally so I would suggest that we revamp the script so that it first activates de environment and it then checks whether cwltool is installed.