Use PySLURM in pull_dicoms_workflow

[tsalo]

@mriedel56 pointed me toward PySLURM on Slack. It would be particularly useful for pull_dicoms_workflow, which is run from a login node and which submits a job for the main conversion workflow (after downloading dicoms). We'd need to revisit our original idea of minimizing dependencies though. We might even need to require Python 3.