dag: create DAG graph for Serial workflows

[tiborsimko]

This issue is similar to #512.

The Serial workflows don't really have complex DAG graph; the workflow is simply linear. But if we do introduce reana-client visualise command for Yadage and CWL, then for consistency's sake we should also generate linear graph for Serial workflows.

This will allow users to have an easy way to create DAG graphs, to use them in documentation, etc.

It could replace our ASCII-art serial diagrams in various REANA demo examples like https://github.com/reanahub/reana-demo-alice-pt-analysis#4-analysis-workflow