a simpler multi-step job submission idea

[gabor-lbl]

A simpler idea for multi-step jobs. Job specs can specify either:

• a list of job steps (each with its own environment, input/output) that would each run via srun
• or, a path to a single, facility-specific job script, to be launched via sbatch

Facility-specific scripts are not ideal because it makes it hard to run these jobs at other facilities.