Draft: New Force Model: Beam Connected Spheres

Hi, I just finished implementing a new force model for YADE. In this model, we can connect spheres with beams in a FEM way. The beams are elastic, support damping, and, if desired, can break according to Mohr-Culomb failure criteria (meaning they behave like brittle materials). They are like brittle materials because there's no plasticity, meaning the material must fail while still on the elastic regime.

I provide several examples and 2 check scripts. The code is also well-documented. There's no paper for this implementation yet, but it will be ready in a couple of months (hopefully).

I tried to make it as performant as I could but if there's a way of making it faster, I'll implement it.

I am eager to think about what you think of the implementation. Anything that can be improved? Is there something else I should add?