Add setup option for multiprocessed jobs with AlphaTwirl

[benkrikler]

This is particularly important when running from a conda installation and is effecting work on lxplus' htcondor.

Steps to implement things:

1. Override alphatwirl's WorkingArea to be able to change the executable. The existing run.py script, the new wrapper script and the user's setup script must all be added to the workingArea tarballs
2. Change the executable provided to htcondor's job description
3. Create a simple executable wrapper that first runs the user's setup script, and then runs the normal run.py
4. Add an option to fast-carpenter to point to the user's setup script.

[benkrikler]

This issue is specific to the alphatwirl backend. It will require a bit of digging around in alphatwirl to implement. With the new Coffea backends I'm not sure how much effort we will be investing there in the future, but I will leave this issue open as a potential road map to implement this.

Note that we now have a mechanism to allow a user to provide this information from the command-line using the recently added execution config file added in PR #129.