Add a slurm workflow manager

[linsword13]

Example usage:

ramble:
  variants:
    workflow_manager: slurm
  variables:
    mpi_command: mpirun -n {n_ranks} -hostfile hostfile
    processes_per_node: 1
  applications:
    hostname:
      workloads:
        local:
          experiments:
            test:
              variables:
                n_nodes: 1

  # The batch_submit is defined by the workflow object
ramble on

  # Executors for query and cancel are available
ramble on --executor "{query_job}"
ramble on --executor "{cancel_job}"

[douglasjacobsen]

I'm not sure how picky I am about this, but I would almost prefer the status map be in python rather than in bash.

And maybe some of the logic mapped into the base class too.

[douglasjacobsen]

We should also think about how to allow this modifier to have a default mpi_command (that could still be overridden)