Aerodynamic drag and J2

[adamdempsey90]

Background

This adds one model of aerodynamic drag to the Nbody particles as well as a J2 component to their potential.

Still draft. TODO

• Test
• Include J2 in particle-particle gravity? Or find out how to add this to rebound if possible.
• Add a spin axis unit vector to the particles and use that in the J2.

Closes #124

Description of Changes

Checklist

• New features are documented
• Tests added for bug fixes and new features
• (@lanl.gov employees) Update copyright on changed files
• Any contribution that was created or modified with the assistance of generative AI must have a comment disclosing this such as // This file was created in part or in whole by generative AI