First usable if incomplete CI

[JoanneBogart]

Exercises two "create" scripts (create_main.py and create_flux.py to a degree by

• invoking each with --help
• invoking each in order to create trilegal catalogs

[JoanneBogart]

Although there is more to do here, in particular in the CI, I would like to merge now so that there is a usable version of the code on the main branch.

For the most part the code has been extracted with little or no change from the skyCatalogs repo. Other than minor clean-up, differences are

• addition of partial exercise of catalog creation in the CI
• fix of a long-standing bug involving overwriting old flux files