Describe your changes

This adds a minor speedup for DirectNBody orbit integration, and adds the ability to use Leapfrog or Ruth integration schemes with the N-body integration functionality.

Checklist

• Did you add tests? - need to parametrize the nbody test to run with different integrators
• Did you add documentation for your changes?
• Did you add a changelog entry? (see CHANGES.rst)
• Are the CI tests passing?
• Is the milestone set?