dag: investigate static DAG generation for CWL workflows

[tiborsimko]

This task is similar to #512 but for CWL workflows rather than Yadage workflows.

Note: see CWL viewer or see cwltool --print-rdf workflow.cwl and other "print" commands.

[mvidalgarcia]

An option would be using the CWL viewer API to download the PNGs.

Some examples:

• https://view.commonwl.org/graph/png/github.com/reanahub/reana-demo-root6-roofit/blob/master/workflow/cwl/workflow.cwl
• https://view.commonwl.org/graph/png/github.com/reanahub/reana-demo-worldpopulation/blob/master/workflow/cwl/worldpopulation-slurmcern.cwl
• https://view.commonwl.org/graph/png/github.com/reanahub/reana-demo-helloworld/blob/master/workflow/cwl/helloworld-htcondorcern.cwl

For some reason it doesn't work for https://view.commonwl.org/graph/png/github.com/reanahub/reana-demo-worldpopulation/blob/master/workflow/cwl/worldpopulation.cwl and https://view.commonwl.org/graph/png/github.com/reanahub/reana-demo-helloworld/blob/master/workflow/cwl/helloworld.cwl

[mr-c]

An option would be using the CWL viewer API to download the PNGs.

For some reason it doesn't work for https://view.commonwl.org/graph/png/github.com/reanahub/reana-demo-worldpopulation/blob/master/workflow/cwl/worldpopulation.cwl and https://view.commonwl.org/graph/png/github.com/reanahub/reana-demo-helloworld/blob/master/workflow/cwl/helloworld.cwl

https://view.commonwl.org/graph/png is not an on-demand endpoint, it only works for already parsed workflows. See https://view.commonwl.org/apidocs