Fix BoundaryModelDummyParticles() for FSI

[LasNikas]

• Incorrect placement of boundary wall for multiple layers (solved in Refactoring setups #42)
• continuity_equation!() dispatched with SummationDensity is needed for the solid-fluid interaction as well. ( Dummy bc for fsi #59 )
• Differentiate movement function for moving boundaries (rework moving boundaries and support for BoundaryModelDummyParticles  #101)
• provide solid particle acceleration for AdamiPressureExtrapolation (Acceleration for tlsph AdamiPressureExtrapolation #262)
• Interaction between different particle spacing (Dummy bc for fsi #59 )
• update solid container in update3!() (Dummy bc for fsi #59 )
• hydrodynamic mass for solid particles (Dummy bc for fsi #59 )

[efaulhaber]

We could just implement setups with analytical density profiles and run the simulation without relaxing part.

Keep in mind that with the higher density at the bottom of the tank, the particles are not just closer together in y-direction, but also in x-direction. In these examples, we can't really see that, but I assume that this is the reason for non-uniform particle distributions. It is therefore not easy to determine the correct positions analytically.

[svchb]

@LasNikas This seems to be a small issue left open. Can you finish this?

[LasNikas]

see #262
Note, that the simulation falling_spheres_2d.jl needs about twice the number of time steps with #262 @svchb @efaulhaber