stdout catching for yadage

In #14 and #15, the standard output of the two steps is being captured in gendata.log and fitdata.log for CWL and Serial workflows.

Let us add the stdout catching for Yadage workflows as well. @lukasheinrich What would be the best paradigm to use? We could use the tee command as for the Serial workflows, but perhaps there exist a more native paradigm like for CWL?