More reshuffling of the sources

Description

Split the content of the src/system folder over several new folders:

• multisystem: now contains all the sources of the multisystem framework
• interactions: existing interactions and the corresponding factory
• output: sources for static output
• classical: classical particles

Also renamed some files to remove the "abst" suffix and renamed some classes: system_abst_t -> system_t, interaction_abst_t -> interaction_t, interaction_gravity_t -> gravity_t, interaction_coulomb_force_t -> coulomb_force_t, and interaction_lorentz_force_t -> lorentz_force_t.

News snippet

More reshuffling of the sources

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.

Closes #354 (closed)