Enable setup scripts to be callable from elsewhere

[aerorahul]

Description
This PR:

• enables user to write their own yaml configuration file for their experiment and call setup_expt.py and setup_xml.py in one shot such as ci/scripts/create_experiment.py. This might be most useful for the GDASApp devs.

Type of change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?
This only changes how the CI is run and the setups for regular users is left unchanged.
Build and creating the experiment dir and XMLs as a test should suffice.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes need updates to the documentation. I have made corresponding changes to the documentation
• My changes generate no new warnings
• New and existing tests pass with my changes
• Any dependent changes have been merged and published