Tidy up finalizers

Description

Tidy up the interactions and systems finalizers:

• Add a system_abst_end routine to free all memory associated with components of systems_abst_t
• Add missing finalizer for ghost_interaction.
• Removed the end method of gravity and lorentz force, as they are not needed.

News snippet

Tidy up finalizers.

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.