Add 4C beam to beam contact

[knarfnitram]

This MR adds the boundary conditions for beam contact with the appropriate header functions.
Additionally, I have added a small test case/ example how a simple example can be set up. (Correct place and extended documentation is open for discussion).

However, I put this currently up as draft to get some feedback, especially since this can only be merged after #160.

[knarfnitram]

Here I have a question:
Currently, I have to add two times the same Boundary Condition.
Wouldn't it make more sense to have something like:
BeamtoBeamContactCondition(nodes1,nodes2)
and under the hood the function create the boundary conditions for nodes1 and nodes2 and keeps track automatically of the IDs ?

[isteinbrecher]

Per definition a boundary condition is always something that acts on a single geometry set in MeshPy. So it is not possible to add two geometry sets to a single BC. What is however possible is to have a factory function or object (similar to what you proposed) what takes beam_contact_bc_factory(mesh, nodes1, nodes2, bc_description, ...) and creates the corresponding BCs automatically. The ID count could either be stored in the factory object, a global variable, or be retrieved by checking how many beam_to_beam_contact conditions are already in the mesh and dividing that number by 2. @knarfnitram @davidrudlstorfer do you have an idea what would be a Pythonic way to track the number of added IDs? I am not sure if global variables and objects are desirable.

[knarfnitram]

So I added a Factory called BeamtoBeamContactBoundaryConditionFactory, which stores the id and adds the geometry sets.
Should we add this factory to the input file? (since the id should be in synch with the input file)