When running in parallel/fork mode, properly clean temporary directories

[kpaulisse]

This PR addresses the problem reported in #146 by eliminating the use of at_exit within parallelized/forked processes. The new design ensures that the temporary directories are properly cleaned, whether parallelization is enabled or not.

Serial execution:

• (TASK: (create temp dir) (do its thing) (remove temp dir at exit))
• (TASK: (create temp dir) (do its thing) (remove temp dir at exit))
• (TASK: (create temp dir) (do its thing) (remove temp dir at exit))
• at_exit: Remove temp dirs

Parallel execution:

• Create temp dir
• Parallel/fork:
  • Task that uses temp dir
  • Task that uses temp dir
  • Task that uses temp dir
• Clean up temp dir