Contact stiffness composite body

[WeiyiVirtonomy]

I added a function Real ContactStiffness(size_t index_i) in Solid. For composite material, it will return composite_materials_[(*material_id_)[index_i]]->ContactStiffness();.

In RepulsionForce, solid_material->ContactStiffness() is replaced by solid_material->ContactStiffness(index_i).

I think the contact impedance of self-contact also needs to be modified. How should be calculate the impedance between particle i and j with different rho and contact stiffness?