Lower the default slurm concurrency limit to 15

[JamesWrigley]

With 30 I still observe some SQLite locking errors, including from the GUI (which crashed it).

[takluyver]

If we're changing the default, let's change it for the CLI as well:

DAMNIT/damnit/cli.py

Lines 112 to 115 in 2548d5e

	reprocess_ap.add_argument(
	'--concurrent-jobs', type=int, default=30,
	help="The maximum number of jobs that will run at once (default 30)"
	)

Other than that, LGTM.

[JamesWrigley]

Roger that, fixed in 5ebdc47.