Dispatch tasks to remote hosts

[jqmp]

Currently, Bionic runs all tasks locally, on the same host it was started on. Ideally it would be able to dispatch jobs to other hosts. (Presumably this would depend on some kind of existing cluster system like Dask.) This would allow flows to exploit very high levels of parallelization, and also enable some workloads that require too much memory for a single machine.

We would almost certainly want to implement #43 before this.