The sign of the body force might be unclear. E.g. in simple_cube the body force is defined positive in positive z or y direction. But the gravitation would work in the opposite direction.
add interface function in experiment base class defining the direction of the gravitation (to be fone in each child)
implement the create_body_force fct globally in the base class once for all
The sign of the body force might be unclear. E.g. in simple_cube the body force is defined positive in positive z or y direction. But the gravitation would work in the opposite direction.