Add new "constant" propagator

Description

Add a new default propagator. This corresponds to the propagator of a system that does not change in time, but still advances in time and interacts with other system.

Note that this is marked as WIP because the last three commits of this branch will have to be rebased onto develop once !979 (merged) and !976 (merged) are merged.

News snippet

Add a new default propagator.

Checklist

• I have checked that my code follows the Octopus coding standards
• I have added tests for all the new features added in this request.