Add CWL + MPI docs

[kinow]

Changelog Entry

To be copied to the draft changelog by merger:

• PR submitter writes their recommendation for a changelog entry here

Add documentation for MPI support in CWL workflows in Toil. Closes #5546

Reviewer Checklist

• Make sure it is coming from issues/XXXX-fix-the-thing in the Toil repo, or from an external repo.
  • If it is coming from an external repo, make sure to pull it in for CI with:

    contrib/admin/test-pr otheruser theirbranchname issues/XXXX-fix-the-thing
    

  • If there is no associated issue, create one.
• Read through the code changes. Make sure that it doesn't have:
  • Addition of trailing whitespace.
  • New variable or member names in camelCase that want to be in snake_case.
  • New functions without type hints.
  • New functions or classes without informative docstrings.
  • Changes to semantics not reflected in the relevant docstrings.
  • New or changed command line options for Toil workflows that are not reflected in docs/running/{cliOptions,cwl,wdl}.rst
  • New features without tests.
• Comment on the lines of code where problems exist with a review comment. You can shift-click the line numbers in the diff to select multiple lines.
• Finish the review with an overall description of your opinion.

Merger Checklist

• Make sure the PR passed tests, including the Gitlab tests, for the most recent commit in its branch.
• Make sure the PR has been reviewed. If not, review it. If it has been reviewed and any requested changes seem to have been addressed, proceed.
• Merge with the Github "Squash and merge" feature.
  • If there are multiple authors' commits, add Co-authored-by to give credit to all contributing authors.
• Copy its recommended changelog entry to the Draft Changelog.
• Append the issue number in parentheses to the changelog entry.

[kinow]

Preview from locally built make docs:

[mr-c]

I don't think toil-cwl-runner actually supports slurm + MPI properly yet

[mr-c]

This means that toil is not correctly configured for MPI. such flags should only be added to MPI jobs, not every single job.

[kinow]

Ah, I think I see what you mean. In this example it'd be fine as there is a single job and it is using MPI. But if we had other jobs that do not require MPI and must be executed elsewhere (cloud, locally, container) then we wouldn't want them to be under the same Slurm allocation.