Create a pid-specific temporary directory and clean it up at the end

[reuterbal]

This expands the gettempdir utility to

1. create pid-specific temporary directory to avoid race conditions among concurrent Loki executions (hoping this resolves the CI failures with OMNI regression testing)
2. clean up the temporary directory when the python interpreter terminates

[github-actions[bot]]

Documentation for this branch can be viewed at https://sites.ecmwf.int/docs/loki/261/index.html