[Regression Test] cube for PMB and single material

[JBludau]

Initial version of a 1mm cube for regression testing

TODOs

• Have a functor for comparing the stresses
• Probably also have a functor for comparing the Poisson ratio
• Both will probably need something to either average (Verlet integration) or ADR integration

[JBludau]

I added a functor that tries to evaluate the displacement error.
The problem seems to be that the particles are wiggling back and forth without the ADR damping and thus comparing displacements of individual particles is not easy. Averaging over the center region of the cube did not help.

@pabloseleson @streeve any thoughts? I don't know if there is a good fix for this except using the ADR damping...

[JBludau]

As per discussion of the last meeting with @pabloseleson and @streeve, this we now prescribe a displacement on all particles that are less than the horizon away from any of the faces of the cube. This should eliminate surface effects.

Nevertheless, I need to allow approximately 10% error on the displacement to have this case pass.
Same holds for the contraction due to the Poisson ratio of 0.25