Reorganize the DAGFlow class

[ikondov]

• Separate the checking functions and the visualizations in DAGFlow class
• Remove the --graph option in lpad check_wflow (or make it strictly alternative to a --check option)
• Restrict the dataflow checks to PyTask and dataflow Firetasks (https://materialsproject.github.io/fireworks/dataflow_tasks.html)
• Make the messages of checks better understandable
• Use more specific exceptions instead of assertions so that the caller can catch them if needed