Grf arx hotfix

[dajmcdon]

Change explanations for reviewer

• Enables the use of grf_quantiles with the arx_forecaster()
• Small change to canned forecaster printing (don't print other_keys when there aren't any).
• Use the epidatasets source in a failing test.

Magic GitHub syntax to mark associated Issue(s) as resolved when this is merged into the default branch

• Resolves #{issue number}