Propagation with different time steps
Propagation with different time-steps for celestial bodies is working, so this MR adds several tests to cover this. Although not needed yet, as all celestial bodies interact with all other celestial bodies, this MR also adds a ghost interaction to be used when one system does not have any physical interaction with another system.
Add ghost interaction and tests for propagation with different time-steps.
- I have checked that my code follows the Octopus coding standards
- I have added tests for all the new features added in this request.