multi-step job submission proposal

[gabor-lbl]

A proposal for running more complex job scripts, using a DAG of steps:

• each step is a slurm "srun" (or similar) with its own optional environment (that overrides the global defintion)
• can optionally declare a parent step (by name)
• Steps with no dependencies run first, then the steps that depend on these.
• Steps run in parallel when they have no dependencies between them.
• Steps run serially when one step depends on another.
• a child step can run optionally based on a condition where it can use the parent step's EXIT_CODE for the test

Note, this proposal does not pretend to be an entire workflow engine but rather a simple (?) way to write cross-platform job scripts with multiple steps.