dag: investigate static DAG generation for Yadage workfows

[tiborsimko]

Investigate how to obtain machine-readable DAG representation for Yadage workflows without actually running them.

This will be useful for (1) static DAG analysis that could be possibly hooked into at the workflow controller level in the future, as well as for (2) a possible new user-facing command reana-client visualise (or reana-client show-graph etc) that will allow users to have a graphical output of their workflow DAG graph with information about inputs, steps, jobs, etc. The visualise command could create e.g. PNG image of the workflow as Yadage currently does.

[lukasheinrich]

one issue is that not all yadage workflows have a static graph, but the graphs are dynamically created depending on the input data, we can try to find a visualization between the dynamic steps