Autoremove run folders once they are not needed

• Simplify (and deduplicate) job lookup by ID.

• cleanUpTaskPath() -> cleanupTaskOutputFilenames() to clarify functionality.

• Separate path building from directory creation.

• Replace exists()/join() pattern with makedirs().

• Guarantee either a JobRecord or None is returned by jobFromId().

• Rename jobsFor() -> nodesFor(), and create a jobsFor() method that yields JobRecords.

• Add documentation where needed.

• Delete unused functions.

Fixes #41 (closed)