[GeoMechanicsApplication] Creation of a backbone model for interface elements

[rfaasse]

For linear truss elements a backbone with un and reloading model is ( being ) created. That shows the mechanism for switching between a backbone and an elastic un and reloading branch.

For interface elements the following is important:

• Interfaces have more ( number of dimensions ) traction and relative displacement components ( as opposed to only 1 axial component for a truss ). The backbone model could work for the normal component of the interface and for a resultant shear component. ( in 2D there is only 1 shear component, which then is the effective one. In 3D 2 shear components together form an effective shear, this makes unloading reloading detection more involved )
• the truss model so far has a backbone that mirrors in positive and negative domain. For tension cutoff on the interface normal behaviour a continuous backbone defined from the negative to the positive domain may be useful.
• material input syntax that distinguishes normal and shear has to be thought trough.
• When "ready" the interface backbone model has to be deployed in the wall interfaces of the tunnel model.

[rfaasse]

@WPK4FEM @mcgicjn2 Moving this back to the backlog, since for the user meeting we're realistically only able to use linear elastic materials for the interfaces for now. After the investigations in the current sprint (18), it might be possible to pick it up again