Replace print_job_status

[pmarchand1]

Addresses #18 (and to a lesser extent #9)

Replaces print_job_status (now deprecated) with get_job_status which returns a list of three elements:

• completed (logical): whether the job completed/stopped,
• queue (data frame): the table from squeue (0 rows if completed),
• log (character vector): the contents of the output file for each node.

Also added a custom print function so that when output directly to console, it shows either the queue table or a message saying all nodes completed, and then outputs the last portion (up to 60 chars.) of the log from each node.