Add debugging support for DAG

[furuame]

It would make modelers' lives easier to debug Event ordering by just enabling scheduler debug logger -l scheduler debug 1.

I implicitly turn on the early cycle detection when the debug logger is enabled. I think that would be useful since that seems never be enabled 🤔 There is a method to turn it on but you would need to re-compile the software during debugging. Let me know your thoughts.

[furuame]

I just saw the test failures. I should keep the method to keep backward compatibility.