Request: Add some sort of clusterID method to `jobsub_api.SubmittedJob` class

[shreyb]

@vhewes requested that we add a method to the jobsub_api.SubmittedJob class that returns the cluster ID without a ProcID.

So something like

job.new_method() --> "12345@schedd.fnal.gov"

(note the lack of ".0" procID)

It's just f"{job.seq}@{job.schedd}", but having a method to return this would make it so users wouldn't have to remember that.

[shreyb]

If this is a DAG - we need to make sure that it's the DAGMANJobID without the proc. That needs to be considered.