[idea] flux.job.FluxExecutor

[jan-janssen]

Currently they implement the following command:

from_command(
        cls,
        command,
        num_tasks=1,
        cores_per_task=1,
        gpus_per_task=None,
        num_nodes=None,
        exclusive=False,
    )

Shell scripts can be submitted to the queuing system and a concurrent.futures.Future object is returned. It would be great to introduce the same abstraction layer for other queuing systems. As the storage interface for such a system is already available in #206 it should not be too complicated to implement.