Updates in preparation for making repo public

[rmandelb]

This is a start at repo updates needed before making the repo public, such as ensuring proper attributions for inputs and clear explanations in the README.

A few questions came up as I was going over the repo:

• Do the installation instructions need to be updated? I realize there is the GalSim question but also not sure if these are correct for EuclidLike-imsim (Edit: there is a separate issue for installation instructions)
• I noticed that the euclidlike-imsim directory includes a bunch of stuff related to SCAs (from filenames and using grep to see what is in the files). Are these residues of roman_imsim that should be cleaned up or removed?

[FedericoBerlfein]

Looks good to me