Support dynamic process creation

[joestubbs]

A very common pattern is to spawn a number of processes ("workers") dependent on the "size" of the output from a previous step (e.g., a dynamic version of the pi approximation wf). There are a number of ways this could work from the client/api perspective. Nextflow has several examples. We should first document what we could pull off using doit. There may be some limitations around using doit with multiprocessing and actions defined using closures.

[deardooley]

Really like a launcher port/alternative here. If we can replicate launcher with service discovery over serfnode, we have MPI in sight as well.